-- =====================================================================
-- lib-core.maps — Single Tile-Grid Map Lib (P.0)
-- See: meta/docs/superpowers/specs/2026-05-09-p0-lib-maps-design.md
--
-- Forward-compat stubs (DEPRECATED-MVP) for: multi-map-graph,
-- lifecycle-states, walls, sprite-fields, generators, save-integration,
-- cross-lib tilemap resolution, override-field-merge, sub-require.
-- =====================================================================

-- Module-private state
local map_registry      = {}    -- map_id -> Map
local tilemap_registry  = {}    -- full_tilemap_id -> Tilemap
local current_map_id    = nil

-- Internal: resolve and return map by id, or raise.
local function require_map(map_id)
    local id = map_id or current_map_id
    if not id then error("maps: no current map set") end
    local m = map_registry[id]
    if not m then error(string.format("maps: no map registered with id '%s'", id)) end
    return m
end

-- =====================================================================
-- Packed-u32 GID encoding (Schema-v2)
-- Bit-Layout:
--   Bits 31..24 : atlas_index  (8 bits, 0..255)
--   Bits 23..4  : tile_id      (20 bits, 0..1_048_575)
--   Bits 3..2   : rotation     (2 bits, 0..3, n * 90° CW)
--   Bits 1..0   : reserved     (must be 0 in v2; future flip-h/v)
-- gid == 0 means empty cell (atlas 0 + tile 0 reserved as null).
-- =====================================================================

local GID_ATLAS_SHIFT = 24
local GID_TILE_SHIFT  = 4
local GID_ROT_SHIFT   = 2
local GID_ATLAS_MASK  = 0xFF        -- 8 bits
local GID_TILE_MASK   = 0xFFFFF     -- 20 bits
local GID_ROT_MASK    = 0x3         -- 2 bits

local function encode_gid(atlas_index, tile_id, rotation)
    rotation = rotation or 0
    return (atlas_index << GID_ATLAS_SHIFT)
         | (tile_id     << GID_TILE_SHIFT)
         | (rotation    << GID_ROT_SHIFT)
end

local function decode_gid(gid)
    local atlas_index = (gid >> GID_ATLAS_SHIFT) & GID_ATLAS_MASK
    local tile_id     = (gid >> GID_TILE_SHIFT)  & GID_TILE_MASK
    local rotation    = (gid >> GID_ROT_SHIFT)   & GID_ROT_MASK
    return atlas_index, tile_id, rotation
end

-- =====================================================================
-- Schema v1→v2→v3 Auto-Upgrade (transparent beim Load)
-- v1: { id, tilemap, size, tiles[], tile_rotations? }
-- v2: { schema_version, id, size, atlases[], layers: { <name>: { tiles[] } }, roof? }
-- v3: v2 + per-layer optional { material, vertices, overrides } for
--     future autotile + force-override paths. tiles[] remains the legacy
--     cell-grid representation; vertices+overrides are forward-compat
--     stubs not yet wired into the renderer (lib v0.5.0a).
-- =====================================================================

local function upgrade_v1_to_v2(v1)
    local atlas_id = v1.tilemap or "default_tilemap"
    local v1_tiles = v1.tiles or {}
    local v1_rots  = v1.tile_rotations or {}

    local gids = {}
    for i = 1, #v1_tiles do
        local tile_id = v1_tiles[i]
        local rot_deg = v1_rots[i] or 0
        local rot_quad = math.floor(rot_deg / 90) % 4
        if tile_id == 0 then
            gids[i] = 0
        else
            gids[i] = encode_gid(0, tile_id, rot_quad)
        end
    end

    return {
        schema_version = 2,
        id   = v1.id,
        size = v1.size,
        atlases = { atlas_id },
        layers = {
            surface = { tiles = gids }
        }
    }
end

-- Schema v2→v3: no-op shape transformation for 0.5.0a. v3 adds three
-- optional per-layer fields (material, vertices, overrides) that the
-- renderer does not yet honour. Just bumps the version so the v3
-- validator + runtime path accept the map.
local function upgrade_v2_to_v3(v2)
    return {
        schema_version = 3,
        id      = v2.id,
        size    = v2.size,
        atlases = v2.atlases,
        layers  = v2.layers,
        roof    = v2.roof,
    }
end

-- =====================================================================
-- Schema-validation helper (used by all version-specific validators)
-- =====================================================================

-- Schema-validation helper: reads required field with type-check.
local function require_field(t, key, expected_type, source)
    local v = t[key]
    if v == nil then
        error(string.format("maps.load: schema violation in %s: missing required field '%s'",
            source, key))
    end
    if type(v) ~= expected_type then
        error(string.format("maps.load: schema violation in %s: field '%s' must be %s, got %s",
            source, key, expected_type, type(v)))
    end
    return v
end

-- =====================================================================
-- 0.5.0e: layer z-index for the opaque-ceiling-cache. Higher = drawn
-- later = on top. Cells whose opaque-ceiling z is GREATER than the
-- current layer's z can be skipped (a higher opaque layer covers
-- whatever this layer would have drawn). Values are 1-based so 0 can
-- mean "no opaque ceiling at this cell". Declared early because
-- cell_is_opaque_on_layer + compute_opaque_ceiling_cache (defined
-- below) reference these as upvalues.
local LAYER_Z = {
    foundation = 1, subsurface = 2, surface    = 3, topsurface = 4,
    lower_wall = 5, wall       = 6, upper_wall = 7, canopy     = 8,
}
local LAYER_ORDER_TOP_DOWN = {
    "canopy", "upper_wall", "wall", "lower_wall",
    "topsurface", "surface", "subsurface", "foundation",
}

-- Dual-Grid Autotile (Schema-v3 vertex-painted layers, 0.5.0c)
--
-- Two grids, offset by half a tile. The PAINT grid (called `vertices`
-- in code, "map-tiles" in design docs) stores material flags at
-- integer positions. The RENDER grid (called `cells`) is offset by
-- (+0.5, +0.5) tile from the paint grid and is where sprites sit.
-- Each render-cell (x, y) spans 4 surrounding paint-tiles, which act
-- as its 4 corner vertices: TL=(x,y), TR=(x+1,y), BL=(x,y+1),
-- BR=(x+1,y+1).
--
-- Material rule ("any-corner"): render-cell (x, y) is material iff
-- any of its 4 corner paint-tiles is painted. A single painted tile
-- thus produces a 2x2 of material render-cells centred on it.
--
-- Bitmask rule (0.5.6, dual-grid native): each render-cell's 8-bit
-- neighbour bitmask (clockwise from N: N, NE, E, SE, S, SW, W, NW)
-- is derived from its own 4 corner paint-tiles. A cardinal bit is
-- set iff at least one of the 2 paint-tiles on that edge is painted;
-- a diagonal bit is set iff the corner paint-tile in that direction
-- is painted. Then blob-gating zeroes any diagonal bit whose 2
-- adjacent cardinal bits are not both set. The gated bitmask
-- resolves through SLOT_LOOKUP into one of the 14 canonical
-- S-V2E2-RM-Blob slots plus the rotation + flip that transforms the
-- canonical sprite into the rendered one.
--
-- Pre-0.5.6 derived the bitmask from the material status of the 8
-- neighbour render-cells, which violated dual-grid semantics: two
-- render-cells with an empty shared edge could see each other as
-- material whenever any unrelated corner of either was painted,
-- producing connected blobs across visually empty paint-tile gaps.
-- =====================================================================

-- Canonical 14 slots, keyed by their lowest-numbered representative
-- bitmask (matches `prototype-blob-geom` slot enumeration §2.2 of the
-- design paper).
local CANONICAL_SLOT_OF_BITMASK = {
    [0x00] = 0,   -- isolated
    [0x01] = 1,   -- end (N)
    [0x05] = 2,   -- corner_open (N+E, no diag)
    [0x07] = 3,   -- corner_full (N+E+NE)
    [0x11] = 4,   -- straight (N+S)
    [0x15] = 5,   -- tee_open (N+E+S, no diags)
    [0x17] = 6,   -- tee_half (N+E+S + NE)
    [0x1F] = 7,   -- tee_full (N+E+S + NE + SE)
    [0x55] = 8,   -- cross_open (4 cardinals)
    [0x57] = 9,   -- cross_q1 (+1 diag NE)
    [0x5F] = 10,  -- cross_q2adj (+NE +SE)
    [0x77] = 11,  -- cross_q2opp (+NE +SW)
    [0x7F] = 12,  -- cross_q3 (+NE +SE +SW)
    [0xFF] = 13,  -- solid
}

-- Rotate the 8-bit bitmask by 90 degrees clockwise. N->E, E->S, S->W,
-- W->N (and diagonals shift one step CW too). In bit terms each bit
-- moves by +2 positions modulo 8.
local function rotate_bitmask_90cw(b)
    return ((b << 2) | (b >> 6)) & 0xFF
end

-- Mirror the 8-bit bitmask across the vertical axis. N stays, S stays,
-- E<->W, NE<->NW, SE<->SW.
local MIRROR_SRC_TO_DST = { [0] = 0, [1] = 7, [2] = 6, [3] = 5,
                            [4] = 4, [5] = 3, [6] = 2, [7] = 1 }
local function mirror_bitmask_h(b)
    local out = 0
    for src = 0, 7 do
        if ((b >> src) & 1) == 1 then
            out = out | (1 << MIRROR_SRC_TO_DST[src])
        end
    end
    return out
end

-- Apply blob-gating: zero each diagonal bit unless both adjacent
-- cardinals are set. NE needs N+E; SE needs S+E; SW needs S+W;
-- NW needs N+W.
local function apply_blob_gating(b)
    local N = (b >> 0) & 1
    local E = (b >> 2) & 1
    local S = (b >> 4) & 1
    local W = (b >> 6) & 1
    local out = b
    if N == 0 or E == 0 then out = out & ~(1 << 1) end
    if S == 0 or E == 0 then out = out & ~(1 << 3) end
    if S == 0 or W == 0 then out = out & ~(1 << 5) end
    if N == 0 or W == 0 then out = out & ~(1 << 7) end
    return out
end

-- Build SLOT_LOOKUP at module-init. For each of the 14 canonical
-- patterns C, walk all 8 D4 transforms (4 rotations x mirror) and
-- record { slot, rot, flip } at the transformed bitmask. The renderer
-- then does a direct bitmask -> draw-params lookup at draw time.
local SLOT_LOOKUP = {}
for canonical_bitmask, slot_index in pairs(CANONICAL_SLOT_OF_BITMASK) do
    for flip = 0, 1 do
        local p_flipped = canonical_bitmask
        if flip == 1 then p_flipped = mirror_bitmask_h(p_flipped) end
        local p = p_flipped
        for rot = 0, 3 do
            if SLOT_LOOKUP[p] == nil then
                SLOT_LOOKUP[p] = { slot = slot_index, rot = rot, flip = flip }
            end
            p = rotate_bitmask_90cw(p)
        end
    end
end

-- Read a vertex flag from a vertex-grid. The grid is (w+1) x (h+1)
-- stored row-major. Out-of-bounds vertices read as 0 (unpainted).
local function vertex_at(vertices, vw, vh, vx, vy)
    if vx < 0 or vx >= vw or vy < 0 or vy >= vh then return 0 end
    local v = vertices[vy * vw + vx + 1]
    if v == nil or v == 0 or v == false then return 0 end
    return 1
end

-- 0.5.7: read the cell-tile material flag at (cx, cy). Returns 0 if
-- layer.cells_material is absent or the entry is unset/false.
local function cells_material_at(cells_material, w, h, cx, cy)
    if cells_material == nil then return 0 end
    if cx < 0 or cx >= w or cy < 0 or cy >= h then return 0 end
    local v = cells_material[cy * w + cx + 1]
    if v == nil or v == 0 or v == false then return 0 end
    return 1
end

-- 0.5.7: lazy-allocate the per-cell material array on a layer. Returns
-- the (now-guaranteed-present) array. Safe to call repeatedly.
local function ensure_cells_material(layer, w, h)
    if layer.cells_material == nil then
        local n = w * h
        local cm = {}
        for i = 1, n do cm[i] = 0 end
        layer.cells_material = cm
    end
    return layer.cells_material
end

-- Any-corner rule + 0.5.7 cell-tile rule: cell (x, y) is material iff
-- (any of its 4 corner vertices is painted) OR (cells_material[x,y] is
-- set). Out-of-grid cells are non-material.
local function cell_has_material(vertices, cells_material, w, h, x, y)
    if x < 0 or x >= w or y < 0 or y >= h then return false end
    if cells_material_at(cells_material, w, h, x, y) ~= 0 then return true end
    local vw = w + 1
    local vh = h + 1
    if vertex_at(vertices, vw, vh, x,     y    ) ~= 0 then return true end
    if vertex_at(vertices, vw, vh, x + 1, y    ) ~= 0 then return true end
    if vertex_at(vertices, vw, vh, x,     y + 1) ~= 0 then return true end
    if vertex_at(vertices, vw, vh, x + 1, y + 1) ~= 0 then return true end
    return false
end

-- Returns the blob-gated 8-bit neighbour bitmask for cell (x, y).
-- Bit layout (clockwise from N, LSB-first): N, NE, E, SE, S, SW, W, NW.
--
-- 0.5.7: per-cell-origin branch. Cells whose material origin is
-- cells_material[x, y] use the classical 47-blob 8-neighbour rule on
-- effective material (vertex-derived OR cell-derived). Vertex-only
-- cells continue to use the FIX-A 4-own-corner rule (v0.5.6 dual-grid
-- semantics, 5 reachable atlas slots).
--
-- Pre-condition: cell_has_material(vertices, cells_material, w, h, x, y)
-- returned true for this cell.
local function compute_cell_bitmask_v3(vertices, cells_material, w, h, x, y)
    local b = 0
    if cells_material_at(cells_material, w, h, x, y) ~= 0 then
        -- 47-blob branch: 8 neighbour effective-material bits.
        if cell_has_material(vertices, cells_material, w, h, x,     y - 1) then b = b | 0x01 end
        if cell_has_material(vertices, cells_material, w, h, x + 1, y - 1) then b = b | 0x02 end
        if cell_has_material(vertices, cells_material, w, h, x + 1, y    ) then b = b | 0x04 end
        if cell_has_material(vertices, cells_material, w, h, x + 1, y + 1) then b = b | 0x08 end
        if cell_has_material(vertices, cells_material, w, h, x,     y + 1) then b = b | 0x10 end
        if cell_has_material(vertices, cells_material, w, h, x - 1, y + 1) then b = b | 0x20 end
        if cell_has_material(vertices, cells_material, w, h, x - 1, y    ) then b = b | 0x40 end
        if cell_has_material(vertices, cells_material, w, h, x - 1, y - 1) then b = b | 0x80 end
    else
        -- FIX-A 4-corner-vertex branch (v0.5.6 dual-grid, unchanged).
        local vw = w + 1
        local vh = h + 1
        local TL = vertex_at(vertices, vw, vh, x,     y    ) ~= 0
        local TR = vertex_at(vertices, vw, vh, x + 1, y    ) ~= 0
        local BL = vertex_at(vertices, vw, vh, x,     y + 1) ~= 0
        local BR = vertex_at(vertices, vw, vh, x + 1, y + 1) ~= 0
        if TL or TR then b = b | 0x01 end  -- N
        if TR       then b = b | 0x02 end  -- NE
        if TR or BR then b = b | 0x04 end  -- E
        if BR       then b = b | 0x08 end  -- SE
        if BL or BR then b = b | 0x10 end  -- S
        if BL       then b = b | 0x20 end  -- SW
        if TL or BL then b = b | 0x40 end  -- W
        if TL       then b = b | 0x80 end  -- NW
    end
    return apply_blob_gating(b)
end

-- Build slot -> tile_id index for an atlas. Tiles named slot_NN_<rest>
-- contribute to the index. Cached on the atlas record to avoid repeat
-- parsing.
local function build_slot_index(atlas)
    local idx = {}
    for tile_id, t in pairs(atlas.tiles) do
        if t.name then
            local slot_str = string.match(t.name, "^slot_(%d+)_")
            if slot_str then
                idx[tonumber(slot_str)] = tile_id
            end
        end
    end
    return idx
end

local function atlas_slot_index(atlas)
    if atlas._slot_index == nil then
        atlas._slot_index = build_slot_index(atlas)
    end
    return atlas._slot_index
end

-- v0.5.1 override-format helper: normalize a stored override entry to
-- {slot, rot, flip}. Disk-compact form is a bare integer slot_id (back-
-- wards-compat with 0.5.0d); object form `{slot, rot?, flip?}` ships
-- rotation+flip explicitly for direct-mode terrain-override placement
-- with non-canonical orientation. Returns nil for malformed input.
local function normalize_override_entry(entry)
    if type(entry) == "number" then
        return { slot = entry, rot = 0, flip = 0 }
    end
    if type(entry) == "table" and type(entry.slot) == "number" then
        return {
            slot = entry.slot,
            rot  = entry.rot  or 0,
            flip = entry.flip or 0,
        }
    end
    return nil
end

-- Predicate "is cell (x, y) opaque on layer_name". Used by the
-- opaque-ceiling cache. Reads the actual atlas tile.opaque flag set
-- by atlas-baker v0.2.0+ alpha-analysis (was previously a slot-13
-- heuristic in 0.5.0e — replaced now that the real data is available).
--
-- Pre-v0.2.0 atlases without the opaque flag set will report all-
-- not-opaque, harmlessly forcing the renderer to draw layers it could
-- have skipped. Safe-conservative.
local function cell_is_opaque_on_layer(map, layer_name, x, y)
    local layer = map.layers[layer_name]
    if not layer then return false end
    if layer.vertices then
        -- Resolve the slot that would render at this cell (override
        -- first, else bitmask), then look up the actual tile.opaque
        -- flag in the layer's material atlas.
        local atlas = layer.material and map.atlas_by_alias
                  and map.atlas_by_alias[layer.material]
        if not atlas then return false end
        local override_entry = layer.overrides and layer.overrides[x .. ":" .. y]
        local slot
        if override_entry ~= nil then
            local norm = normalize_override_entry(override_entry)
            if not norm then return false end
            slot = norm.slot
        else
            if not cell_has_material(layer.vertices, layer.cells_material, map.size.w, map.size.h, x, y) then
                return false
            end
            local bitmask = compute_cell_bitmask_v3(layer.vertices, layer.cells_material, map.size.w, map.size.h, x, y)
            local rec = SLOT_LOOKUP[bitmask]
            if not rec then return false end
            slot = rec.slot
        end
        local tile_id = atlas_slot_index(atlas)[slot]
        local tile = tile_id and atlas.tiles[tile_id]
        return tile and tile.opaque == true or false
    end
    if layer.tiles then
        local gid = layer.tiles[y * map.size.w + x + 1] or 0
        if gid == 0 then return false end
        local atlas_idx, tile_id, _rot = decode_gid(gid)
        local atlas = map.atlases[atlas_idx + 1]
        if not atlas then return false end
        local tile = atlas.tiles[tile_id]
        return tile and tile.opaque == true or false
    end
    return false
end

-- Compute per-cell opaque-ceiling. For each cell, return the z-index
-- of the topmost layer with an opaque tile, or 0 for no ceiling.
local function compute_opaque_ceiling_cache(map)
    local cells = map.size.w * map.size.h
    local cache = {}
    for i = 1, cells do cache[i] = 0 end
    for _, layer_name in ipairs(LAYER_ORDER_TOP_DOWN) do
        local z = LAYER_Z[layer_name]
        for y = 0, map.size.h - 1 do
            for x = 0, map.size.w - 1 do
                local idx = y * map.size.w + x + 1
                if cache[idx] == 0
                   and cell_is_opaque_on_layer(map, layer_name, x, y) then
                    cache[idx] = z
                end
            end
        end
    end
    return cache
end

-- Lazily compute (or return cached) opaque-ceiling for a map. Writes
-- nil out the cache; the next render call rebuilds it.
local function ensure_opaque_ceiling(map)
    if map._opaque_ceiling == nil then
        map._opaque_ceiling = compute_opaque_ceiling_cache(map)
    end
    return map._opaque_ceiling
end

-- =====================================================================
-- Schema-v2 Layer-Whitelist + Validation
-- =====================================================================

-- Layer z-order (back-to-front) split at entity slot
local LAYER_ORDER_PRE_ENTITIES  = { "foundation", "subsurface", "surface", "topsurface" }
local LAYER_ORDER_POST_ENTITIES = { "lower_wall", "wall", "upper_wall", "canopy" }

local VALID_LAYER_NAMES = {
    foundation  = true,
    subsurface  = true,
    surface     = true,
    topsurface  = true,
    lower_wall  = true,
    wall        = true,
    upper_wall  = true,
    canopy      = true,
}

-- (Moved up earlier — see autotile section)

local function validate_map_table_v2(t, source)
    require_field(t, "schema_version", "number", source)
    if t.schema_version ~= 2 then
        error(string.format("maps.load: schema_version %d not supported (expected 2)",
            t.schema_version))
    end
    require_field(t, "id", "string", source)
    local size = require_field(t, "size", "table", source)
    require_field(size, "w", "number", source .. ".size")
    require_field(size, "h", "number", source .. ".size")
    local expected = size.w * size.h

    local atlases = require_field(t, "atlases", "table", source)
    if #atlases == 0 then
        error(string.format("maps.load: schema violation in %s: atlases[] must be non-empty", source))
    end
    if #atlases > 256 then
        error(string.format("maps.load: schema violation in %s: atlases[] has %d entries, max 256", source, #atlases))
    end

    local layers = t.layers or {}
    for layer_name, layer_data in pairs(layers) do
        if not VALID_LAYER_NAMES[layer_name] then
            engine.print(string.format("maps.load: %s: ignoring unknown layer '%s'", source, layer_name))
        else
            local tiles = require_field(layer_data, "tiles", "table",
                source .. ".layers." .. layer_name)
            if #tiles ~= expected then
                error(string.format("maps.load: schema violation in %s.layers.%s: tiles length %d != %d",
                    source, layer_name, #tiles, expected))
            end
        end
    end

    if t.roof ~= nil then
        if type(t.roof) ~= "table" then
            error(string.format("maps.load: %s: roof must be array", source))
        end
        if #t.roof ~= expected then
            error(string.format("maps.load: %s: roof length %d != %d", source, #t.roof, expected))
        end
    end
end

-- v3 validator. Same as v2 except:
--   * schema_version must be 3
--   * per-layer 'material' (string), 'vertices' (array), 'overrides' (table)
--     are optional. If present they must be the right type. Renderer does
--     not yet honour them (forward-compat for 0.5.0c/d).
local function validate_map_table_v3(t, source)
    require_field(t, "schema_version", "number", source)
    if t.schema_version ~= 3 then
        error(string.format("maps.load: schema_version %d not supported (expected 3)",
            t.schema_version))
    end
    require_field(t, "id", "string", source)
    local size = require_field(t, "size", "table", source)
    require_field(size, "w", "number", source .. ".size")
    require_field(size, "h", "number", source .. ".size")
    local expected = size.w * size.h

    local atlases = require_field(t, "atlases", "table", source)
    if #atlases == 0 then
        error(string.format("maps.load: schema violation in %s: atlases[] must be non-empty", source))
    end
    if #atlases > 256 then
        error(string.format("maps.load: schema violation in %s: atlases[] has %d entries, max 256", source, #atlases))
    end

    local layers = t.layers or {}
    for layer_name, layer_data in pairs(layers) do
        if not VALID_LAYER_NAMES[layer_name] then
            engine.print(string.format("maps.load: %s: ignoring unknown layer '%s'", source, layer_name))
        else
            local lsrc = source .. ".layers." .. layer_name
            -- tiles[] is the legacy cell-grid representation (kept from v2).
            -- Required for any non-empty layer in 0.5.0a (autotile path comes in 0.5.0c).
            if layer_data.tiles ~= nil then
                if type(layer_data.tiles) ~= "table" then
                    error(string.format("maps.load: schema violation in %s: tiles must be array", lsrc))
                end
                if #layer_data.tiles ~= expected then
                    error(string.format("maps.load: schema violation in %s: tiles length %d != %d",
                        lsrc, #layer_data.tiles, expected))
                end
            end
            -- Forward-compat fields (0.5.0c/d will wire them into the renderer).
            if layer_data.material ~= nil and type(layer_data.material) ~= "string" then
                error(string.format("maps.load: schema violation in %s: material must be string", lsrc))
            end
            if layer_data.vertices ~= nil and type(layer_data.vertices) ~= "table" then
                error(string.format("maps.load: schema violation in %s: vertices must be array", lsrc))
            end
            -- 0.5.7: optional cells_material (W*H bool array, lazy-allocated)
            if layer_data.cells_material ~= nil then
                if type(layer_data.cells_material) ~= "table" then
                    error(string.format("maps.load: schema violation in %s: layer '%s' field 'cells_material' must be table, got %s",
                        source, layer_name, type(layer_data.cells_material)))
                end
                local expected_cm = size.w * size.h
                if #layer_data.cells_material ~= expected_cm then
                    error(string.format("maps.load: schema violation in %s: layer '%s' field 'cells_material' length %d != W*H %d",
                        source, layer_name, #layer_data.cells_material, expected_cm))
                end
            end
            if layer_data.overrides ~= nil then
                if type(layer_data.overrides) ~= "table" then
                    error(string.format("maps.load: schema violation in %s: overrides must be table", lsrc))
                end
                -- v0.5.1: each override entry is either a bare int (compact
                -- canonical-orientation form) or an object {slot, rot?, flip?}
                -- for explicit-transform placement. Validate per-entry.
                for k, v in pairs(layer_data.overrides) do
                    if type(v) == "number" then
                        if v ~= math.floor(v) or v < 0 or v > 13 then
                            error(string.format(
                                "maps.load: schema violation in %s.overrides[%s]: bare slot %s must be integer 0..13",
                                lsrc, tostring(k), tostring(v)))
                        end
                    elseif type(v) == "table" then
                        if type(v.slot) ~= "number" or v.slot ~= math.floor(v.slot)
                           or v.slot < 0 or v.slot > 13 then
                            error(string.format(
                                "maps.load: schema violation in %s.overrides[%s]: object must have slot integer 0..13",
                                lsrc, tostring(k)))
                        end
                        if v.rot ~= nil and (type(v.rot) ~= "number" or v.rot < 0 or v.rot > 3) then
                            error(string.format(
                                "maps.load: schema violation in %s.overrides[%s]: rot must be 0..3",
                                lsrc, tostring(k)))
                        end
                        if v.flip ~= nil and v.flip ~= 0 and v.flip ~= 1 then
                            error(string.format(
                                "maps.load: schema violation in %s.overrides[%s]: flip must be 0 or 1",
                                lsrc, tostring(k)))
                        end
                    else
                        error(string.format(
                            "maps.load: schema violation in %s.overrides[%s]: value must be integer slot or {slot, rot?, flip?} object",
                            lsrc, tostring(k)))
                    end
                end
            end
            -- Reserved per-layer fields (design paper §8 + §14b + §15.1).
            -- Accept-but-ignore today; future slices wire them into the
            -- renderer / collision-resolver / movement-mode system.
            -- Type-checks here catch malformed values early instead of
            -- letting them survive as silent garbage.
            if layer_data.base_color ~= nil and type(layer_data.base_color) ~= "string" then
                error(string.format("maps.load: schema violation in %s: base_color must be string (#RRGGBB)", lsrc))
            end
            if layer_data.tint_override ~= nil and type(layer_data.tint_override) ~= "table" then
                error(string.format("maps.load: schema violation in %s: tint_override must be table", lsrc))
            end
            if layer_data.collision_policy ~= nil and type(layer_data.collision_policy) ~= "table" then
                error(string.format("maps.load: schema violation in %s: collision_policy must be table", lsrc))
            end
            if layer_data.traversal_modes ~= nil and type(layer_data.traversal_modes) ~= "table" then
                error(string.format("maps.load: schema violation in %s: traversal_modes must be table", lsrc))
            end
            if layer_data.foundation_mode ~= nil and type(layer_data.foundation_mode) ~= "table" then
                error(string.format("maps.load: schema violation in %s: foundation_mode must be array (per-cell 0/1)", lsrc))
            end
        end
    end

    -- Reserved map-level fields for §14c multi-z-level support.
    -- Accept-but-ignore today; future lib-core.maps v0.6 wires them
    -- into a recursive load+link + foundation-reveal-composite pipeline.
    if t.z_level ~= nil and type(t.z_level) ~= "number" then
        error(string.format("maps.load: schema violation in %s: z_level must be integer", source))
    end
    if t.z_below ~= nil and type(t.z_below) ~= "string" then
        error(string.format("maps.load: schema violation in %s: z_below must be string map-id reference", source))
    end

    if t.roof ~= nil then
        if type(t.roof) ~= "table" then
            error(string.format("maps.load: %s: roof must be array", source))
        end
        if #t.roof ~= expected then
            error(string.format("maps.load: %s: roof length %d != %d", source, #t.roof, expected))
        end
    end
end

-- =====================================================================
-- Internal helpers
-- =====================================================================

-- Checks whether `full_id` is a tilemap belonging to the current module.
-- Module-IDs can themselves contain dots (e.g. `lib-core.maps-test`), so a
-- naive first-dot-split is wrong. Match by prefix `<current-module-id>.`.
-- Returns (is_local, local_name) — local_name is nil if not local.
local function split_local_tilemap(full_id)
    local mod = engine.module.id()
    local prefix = mod .. "."
    if string.sub(full_id, 1, #prefix) == prefix then
        return true, string.sub(full_id, #prefix + 1)
    end
    return false, nil
end

-- "demo_tilemap"           -> "<current-module-id>.demo_tilemap"
-- "lib-x.foo"              -> "lib-x.foo" (cross-lib path; checked at load)
local function resolve_tilemap_id(ref)
    if string.find(ref, ".", 1, true) then
        return ref
    end
    return engine.module.id() .. "." .. ref
end

local function validate_map_table(t, source)
    require_field(t, "id", "string", source)
    require_field(t, "tilemap", "string", source)
    local size = require_field(t, "size", "table", source)
    require_field(size, "w", "number", source .. ".size")
    require_field(size, "h", "number", source .. ".size")
    local tiles = require_field(t, "tiles", "table", source)
    local expected = size.w * size.h
    if #tiles ~= expected then
        error(string.format("maps.load: schema violation in %s: tiles array length %d != size.w * size.h (%d)",
            source, #tiles, expected))
    end
end

local function validate_tilemap_table(t, source, expected_local_name)
    require_field(t, "id", "string", source)
    if t.id ~= expected_local_name then
        error(string.format("maps.load: tilemap manifest id '%s' mismatches filename-stem '%s' in %s",
            t.id, expected_local_name, source))
    end
    require_field(t, "tile_size", "number", source)
    local tiles = require_field(t, "tiles", "table", source)
    for i, entry in ipairs(tiles) do
        if type(entry) ~= "table" then
            error(string.format("maps.load: tilemap %s tiles[%d] must be table", source, i))
        end
        require_field(entry, "id", "string", source .. ".tiles[" .. i .. "]")
        require_field(entry, "walkable", "boolean", source .. ".tiles[" .. i .. "]")
    end
end

local function load_tilemap(full_id)
    if tilemap_registry[full_id] then
        return tilemap_registry[full_id]
    end
    local is_local, local_name = split_local_tilemap(full_id)
    if not is_local then
        -- DEPRECATED-MVP: cross-lib tilemap resolution deferred to render-slice
        error(string.format("maps.load: cross-lib tilemap resolution deferred [DEPRECATED-MVP]; tilemap '%s' not from current module '%s'",
            full_id, engine.module.id()))
    end
    -- Try legacy tilemap JSON first (M.1 path; present in pre-M.2 assets).
    -- Fall back to an atlas-bootstrap stub when the file is absent (M.2 path):
    -- load_textures will replace this stub with the real atlas data.
    local path = "assets/tiles/" .. local_name .. ".tilemap.json"
    local ok, raw = pcall(engine.asset.load_json, path)
    if ok then
        validate_tilemap_table(raw, path, local_name)
        -- Cook tiles: copy raw fields and add texture + texture_handle slots.
        local cooked = {}
        for i, t in ipairs(raw.tiles) do
            cooked[i] = {
                id             = t.id,
                walkable       = (t.walkable == true),
                color          = t.color,        -- color-mode fallback
                texture        = t.texture,      -- optional atlas-id
                texture_handle = nil,            -- populated by load_textures
            }
        end
        local tilemap = {
            id         = full_id,
            tile_size  = raw.tile_size,
            asset_pack = raw.asset_pack,         -- optional alias-key
            tiles      = cooked,
        }
        tilemap_registry[full_id] = tilemap
        return tilemap
    end
    -- M.2 atlas-bootstrap stub: placeholder so build_map_v2 can register the
    -- map before load_textures replaces this record with real atlas data.
    -- GID validation is intentionally relaxed (large stub palette).
    engine.print(string.format(
        "maps.load: tilemap JSON not found (%s); using atlas-bootstrap stub for '%s' (call load_textures to populate)",
        path, full_id))
    local stub_tiles = {}
    for i = 1, 4096 do stub_tiles[i] = { id = i, name = "", walkable = false } end
    local stub = {
        id        = full_id,
        tile_size = 32,
        tiles     = stub_tiles,
    }
    tilemap_registry[full_id] = stub
    return stub
end

local function build_map(t_map, tilemap)
    -- Verify each tile-id is in palette range
    for i, tid in ipairs(t_map.tiles) do
        if type(tid) ~= "number" or tid < 1 or tid > #tilemap.tiles then
            error(string.format("maps.load: tile-id %s at index %d exceeds palette size %d (in map '%s')",
                tostring(tid), i, #tilemap.tiles, t_map.id))
        end
    end
    return {
        id             = t_map.id,
        size           = t_map.size,
        tile_size      = t_map.tile_size or tilemap.tile_size,
        tiles          = t_map.tiles,           -- shallow-ref
        tile_rotations = t_map.tile_rotations,  -- optional parallel array (NEW)
        tilemap        = tilemap,               -- shallow-ref
        -- DEPRECATED-MVP: forward-compat stubs (multi-map slice fills)
        walls          = {},
        regions        = {},
        edges          = {},
        state          = "Active",
        pinned         = false,
    }
end

local function build_map_v2(t_map, resolved_atlases)
    local size = t_map.size
    -- Validate that each non-zero GID references a tile_id within its atlas's palette.
    local layers = t_map.layers or {}
    for layer_name, layer_data in pairs(layers) do
        if VALID_LAYER_NAMES[layer_name] and layer_data.tiles then
            for i, gid in ipairs(layer_data.tiles) do
                if gid ~= 0 then
                    local atlas_idx, tile_id, _rot = decode_gid(gid)
                    local atlas = resolved_atlases[atlas_idx + 1]
                    if atlas then
                        if tile_id < 1 or tile_id > #atlas.tiles then
                            error(string.format(
                                "maps.load: tile-id %d at index %d exceeds palette size %d (in map '%s', layer '%s')",
                                tile_id, i, #atlas.tiles, t_map.id or "<unnamed>", layer_name))
                        end
                    end
                end
            end
        end
    end
    return {
        id              = t_map.id,
        schema_version  = 2,
        size            = size,
        tile_size       = resolved_atlases[1].tile_size,  -- assume uniform; first atlas wins
        atlases         = resolved_atlases,
        atlas_aliases   = t_map.atlases,  -- alias strings, parallel to atlases
        layers          = layers,
        roof            = t_map.roof,
        -- DEPRECATED-MVP stubs
        walls   = {},
        regions = {},
        edges   = {},
        state   = "Active",
        pinned  = false,
    }
end

-- v3 runtime build: structurally identical to v2 today (tiles[] is still
-- the cell-grid representation). material/vertices/overrides per layer are
-- preserved on the runtime record but not yet consumed by the renderer.
local function build_map_v3(t_map, resolved_atlases)
    local size = t_map.size
    local layers = t_map.layers or {}
    for layer_name, layer_data in pairs(layers) do
        if VALID_LAYER_NAMES[layer_name] and layer_data.tiles then
            for i, gid in ipairs(layer_data.tiles) do
                if gid ~= 0 then
                    local atlas_idx, tile_id, _rot = decode_gid(gid)
                    local atlas = resolved_atlases[atlas_idx + 1]
                    if atlas then
                        if tile_id < 1 or tile_id > #atlas.tiles then
                            error(string.format(
                                "maps.load: tile-id %d at index %d exceeds palette size %d (in map '%s', layer '%s')",
                                tile_id, i, #atlas.tiles, t_map.id or "<unnamed>", layer_name))
                        end
                    end
                end
            end
        end
    end
    -- Build alias->atlas lookup for the v3 autotile path (layer.material
    -- field is an atlas-alias string).
    local atlas_by_alias = {}
    for i, alias in ipairs(t_map.atlases or {}) do
        atlas_by_alias[alias] = resolved_atlases[i]
    end
    -- 0.5.0e: precompute per-layer has-content bitmap so the renderer can
    -- skip layers that are present in the map JSON but contain no
    -- non-empty data. Layers absent from t_map.layers are simply not in
    -- this dict (effectively false).
    local layer_has_content = {}
    local expected_cells = size.w * size.h
    for layer_name, layer_data in pairs(layers) do
        if VALID_LAYER_NAMES[layer_name] then
            local has = false
            if layer_data.tiles then
                for i = 1, expected_cells do
                    if (layer_data.tiles[i] or 0) ~= 0 then has = true; break end
                end
            end
            if not has and layer_data.vertices then
                for i = 1, #layer_data.vertices do
                    local v = layer_data.vertices[i]
                    if v == 1 or v == true then has = true; break end
                end
            end
            if not has and layer_data.overrides then
                for _ in pairs(layer_data.overrides) do has = true; break end
            end
            layer_has_content[layer_name] = has
        end
    end
    return {
        id              = t_map.id,
        schema_version  = 3,
        size            = size,
        tile_size       = resolved_atlases[1].tile_size,
        atlases         = resolved_atlases,
        atlas_aliases   = t_map.atlases,
        atlas_by_alias  = atlas_by_alias,
        layers          = layers,  -- shallow-ref; preserves material/vertices/overrides if present
        _layer_has_content = layer_has_content,
        roof            = t_map.roof,
        walls   = {},
        regions = {},
        edges   = {},
        state   = "Active",
        pinned  = false,
    }
end

-- =====================================================================
-- Internal render helpers (draw_layer, draw_v1_legacy)
-- These must be declared before the public draw_map* functions that call them.
-- =====================================================================

-- Internal: draw all cells in one layer using atlas-handle + UV sampling.
-- (M.2: replaces per-tile texture_handle pattern with atlas-level handle.)
-- Magenta-placeholder color (RGBA8888 0xFF00FFFF) for missing-asset draw paths.
local MISSING_ASSET_COLOR = 0xFFFF00FF

local function draw_layer(m, layer_name)
    local layer = m.layers[layer_name]
    if not layer then return end
    local sz = m.size
    local ts = m.tile_size
    -- 0.5.0e: opaque-ceiling cache lets us skip cells where a higher
    -- layer fully covers what would be drawn here. Lazily built on
    -- first render after load or after a write invalidates it.
    -- TEMPORARILY DISABLED while debugging vagrant nil-index regression.
    local layer_z = LAYER_Z[layer_name]
    local ceiling = layer_z and ensure_opaque_ceiling(m) or nil

    -- v3 autotile path: layer has a vertex grid + a material atlas
    -- reference. Each cell's tile + transform is computed from the
    -- 8-bit blob bitmask against the canonical 47->14 lookup. Empty
    -- cells (no-corner-painted) skip rendering entirely. Sparse
    -- overrides take precedence over the bitmask path (0.5.0d) and
    -- also force material-presence at the cell even when the vertex
    -- grid would otherwise leave it empty.
    if layer.vertices and layer.material then
        local atlas = m.atlas_by_alias and m.atlas_by_alias[layer.material]
        local overrides = layer.overrides
        if atlas == nil or atlas.diffuse_texture_handle == nil then
            for y = 0, sz.h - 1 do
                for x = 0, sz.w - 1 do
                    local has_ovr = overrides and overrides[x .. ":" .. y] ~= nil
                    if has_ovr or cell_has_material(layer.vertices, layer.cells_material, sz.w, sz.h, x, y) then
                        engine.render.draw_rect(x * ts, y * ts, ts, ts, MISSING_ASSET_COLOR)
                    end
                end
            end
            return
        end
        local slot_idx = atlas_slot_index(atlas)
        for y = 0, sz.h - 1 do
            for x = 0, sz.w - 1 do
                local idx = y * sz.w + x + 1
                if ceiling and layer_z and ceiling[idx] > layer_z then
                    goto continue  -- higher opaque layer covers this cell
                end
                local override_entry = overrides and overrides[x .. ":" .. y]
                local has_material = override_entry ~= nil
                                  or cell_has_material(layer.vertices, layer.cells_material, sz.w, sz.h, x, y)
                if not has_material then
                    goto continue
                end
                local slot_index, rot, flip
                if override_entry ~= nil then
                    -- v0.5.1: override entry can be bare int (canonical
                    -- orientation, backwards-compat) or {slot, rot, flip}
                    -- object for explicit transform placement.
                    local norm = normalize_override_entry(override_entry)
                    if norm then
                        slot_index, rot, flip = norm.slot, norm.rot, norm.flip
                    end
                else
                    local bitmask = compute_cell_bitmask_v3(layer.vertices, layer.cells_material, sz.w, sz.h, x, y)
                    local rec = SLOT_LOOKUP[bitmask]
                    if rec then
                        slot_index, rot, flip = rec.slot, rec.rot, rec.flip
                    end
                end
                local tile_id = slot_index and slot_idx[slot_index] or nil
                local tile = tile_id and atlas.tiles[tile_id] or nil
                local px = x * ts
                local py = y * ts
                if tile and tile.uv then
                    local rot_deg = rot * 90.0
                    local scale_x = ts / tile.uv.w
                    local scale_y = ts / tile.uv.h
                    if flip == 1 then scale_x = -scale_x end
                    engine.render.draw_sprite_transform(
                        atlas.diffuse_texture_handle,
                        px + ts / 2, py + ts / 2,
                        math.rad(rot_deg),
                        scale_x, scale_y,
                        ts / 2, ts / 2,
                        0xFFFFFFFF,
                        tile.uv.x, tile.uv.y, tile.uv.w, tile.uv.h
                    )
                else
                    engine.render.draw_rect(px, py, ts, ts, MISSING_ASSET_COLOR)
                end
                ::continue::
            end
        end
        return
    end

    -- Legacy v2 tiles[] path (cell-grid of packed-u32 GIDs).
    if not layer.tiles then return end
    for y = 0, sz.h - 1 do
        for x = 0, sz.w - 1 do
            local idx = y * sz.w + x + 1
            if ceiling and layer_z and ceiling[idx] > layer_z then
                goto tiles_continue  -- 0.5.0e opaque-ceiling skip
            end
            local gid = layer.tiles[idx] or 0
            if gid ~= 0 then
                local atlas_idx, tile_id, rot_quad = decode_gid(gid)
                local atlas = m.atlases[atlas_idx + 1]
                local px = x * ts
                local py = y * ts
                if atlas and atlas.diffuse_texture_handle then
                    local tile = atlas.tiles[tile_id]
                    if tile and tile.uv then
                        local rot_deg = rot_quad * 90.0
                        engine.render.draw_sprite_transform(
                            atlas.diffuse_texture_handle,
                            px + ts / 2, py + ts / 2,
                            math.rad(rot_deg),
                            ts / tile.uv.w, ts / tile.uv.h,
                            ts / 2, ts / 2,
                            0xFFFFFFFF,
                            tile.uv.x, tile.uv.y, tile.uv.w, tile.uv.h
                        )
                    else
                        -- Tile-record missing or no uv: magenta placeholder.
                        engine.render.draw_rect(px, py, ts, ts, MISSING_ASSET_COLOR)
                    end
                else
                    -- Atlas not loaded (load_textures was never called, or asset
                    -- alias resolution returned nil): magenta placeholder so the
                    -- consumer sees the missing-asset visually instead of crashing.
                    engine.render.draw_rect(px, py, ts, ts, MISSING_ASSET_COLOR)
                end
            end
            ::tiles_continue::
        end
    end
end

local function draw_v1_legacy(m)
    local tm = m.tilemap
    if not tm then return end
    local ts = tm.tile_size
    for y = 0, m.size.h - 1 do
        for x = 0, m.size.w - 1 do
            local idx = y * m.size.w + x + 1
            local tile_id = m.tiles[idx]
            local tile = tm.tiles[tile_id]
            local rot_deg = (m.tile_rotations and m.tile_rotations[idx]) or 0
            local px = x * ts
            local py = y * ts
            if tile.texture_handle then
                engine.render.draw_sprite_transform(
                    tile.texture_handle,
                    px + ts / 2, py + ts / 2,
                    math.rad(rot_deg),
                    1.0, 1.0,
                    ts / 2, ts / 2,
                    0xFFFFFFFF
                )
            else
                local c = tile.color or { 100, 100, 100 }
                engine.render.draw_rect(px, py, ts, ts,
                    engine.render.rgb(c[1], c[2], c[3]))
            end
        end
    end
end

-- =====================================================================
-- Public API
-- =====================================================================

local M = {}

function M.load(path)
    local raw = engine.asset.load_json(path)
    if raw.schema_version == nil or raw.schema_version == 1 then
        raw = upgrade_v1_to_v2(raw)
        engine.print(string.format("maps.load: auto-upgraded v1 map '%s' to v2", raw.id or "<unnamed>"))
    end
    if raw.schema_version == 2 then
        raw = upgrade_v2_to_v3(raw)
        engine.print(string.format("maps.load: auto-upgraded v2 map '%s' to v3", raw.id or "<unnamed>"))
    end
    validate_map_table_v3(raw, path)

    -- Resolve atlas-aliases (each entry is a tilemap-id, possibly local or fully-qualified)
    local resolved_atlases = {}
    for i, alias in ipairs(raw.atlases) do
        local full_id = resolve_tilemap_id(alias)
        resolved_atlases[i] = load_tilemap(full_id)
    end

    local map = build_map_v3(raw, resolved_atlases)
    if map_registry[map.id] then
        error(string.format("maps.load: map-id '%s' already registered", map.id))
    end
    map_registry[map.id] = map
    return map.id
end

-- Programmatic creation (tests, future procedural-map generators).
-- Caller must provide a fully-built tilemap-table (not a path/id ref).
function M.create(t)
    if type(t.tilemap_table) ~= "table" then
        error("maps.create: tilemap_table required (use maps.load for JSON path)")
    end
    local map = build_map(t, t.tilemap_table)
    if map_registry[map.id] then
        error(string.format("maps.create: map-id '%s' already registered", map.id))
    end
    map_registry[map.id] = map
    return map.id
end

function M.size(map_id)
    local id = map_id or current_map_id
    if not id then error("maps.size: no current map") end
    return map_registry[id].size
end

function M.tile_size(map_id)
    local id = map_id or current_map_id
    if not id then error("maps.tile_size: no current map") end
    return map_registry[id].tile_size
end

function M.tile_at_layer(layer_name, x, y, map_id)
    local id = map_id or current_map_id
    if not id then error("maps.tile_at_layer: no current map") end
    local m = map_registry[id]
    if x < 0 or x >= m.size.w or y < 0 or y >= m.size.h then return nil end
    if not m.layers or not m.layers[layer_name] then return nil end
    local gid = m.layers[layer_name].tiles[y * m.size.w + x + 1] or 0
    if gid == 0 then return nil end
    local atlas_idx, tile_id, _rot = decode_gid(gid)
    local atlas = m.atlases[atlas_idx + 1]
    if not atlas then return nil end
    return atlas.tiles[tile_id]
end

-- Arity-flex sugar: tile_at(tx, ty) uses current_map_id; tile_at(map_id, tx, ty)
-- is explicit. Both forms accept nil map_id and fall back to current_map_id.
function M.tile_at(a, b, c)
    local map_id, tx, ty
    if c == nil then
        map_id, tx, ty = current_map_id, a, b
    else
        map_id, tx, ty = a, b, c
        if map_id == nil then map_id = current_map_id end
    end
    if not map_id then
        error("maps.tile_at: no current map; call set_current() first or pass map_id")
    end
    local m = map_registry[map_id]
    if not m then
        error(string.format("maps.tile_at: unknown map-id '%s'", tostring(map_id)))
    end
    -- v2 path: query surface layer for back-compat
    if m.schema_version >= 2 then
        return M.tile_at_layer("surface", tx, ty, map_id)
    end
    -- v1 path (legacy): kept for any non-upgraded maps that bypass M.load
    if tx < 0 or tx >= m.size.w or ty < 0 or ty >= m.size.h then return nil end
    local palette_id = m.tiles[ty * m.size.w + tx + 1]
    return m.tilemap.tiles[palette_id]
end

-- v2 walkability: surface non-empty AND no blocking wall
function M.is_walkable(a, b, c)
    local map_id, tx, ty
    if c == nil then
        map_id, tx, ty = current_map_id, a, b
    else
        map_id, tx, ty = a, b, c
        if map_id == nil then map_id = current_map_id end
    end
    if not map_id then
        error("maps.is_walkable: no current map")
    end
    local m = map_registry[map_id]
    if m.schema_version < 2 then
        -- v1 fallback: surface-only walkability via legacy tile_at
        local t = M.tile_at(map_id, tx, ty)
        if t == nil then return false end
        return t.walkable == true
    end
    -- v2: needs surface tile present AND lower_wall/wall absent
    local surface = M.tile_at_layer("surface", tx, ty, map_id)
    if surface == nil then return false end
    if M.cell_gid("lower_wall", tx, ty, map_id) ~= 0 then return false end
    if M.cell_gid("wall",       tx, ty, map_id) ~= 0 then return false end
    return surface.walkable == true
end

function M.blocks_walk(x, y, map_id)
    local id = map_id or current_map_id
    if not id then error("maps.blocks_walk: no current map") end
    local m = map_registry[id]
    if x < 0 or x >= m.size.w or y < 0 or y >= m.size.h then return false end
    if M.cell_gid("lower_wall", x, y, id) ~= 0 then return true end
    if M.cell_gid("wall",       x, y, id) ~= 0 then return true end
    return false
end

function M.blocks_sight(x, y, map_id)
    local id = map_id or current_map_id
    if not id then error("maps.blocks_sight: no current map") end
    local m = map_registry[id]
    if x < 0 or x >= m.size.w or y < 0 or y >= m.size.h then return false end
    if M.cell_gid("wall", x, y, id) ~= 0 then return true end
    -- upper_wall: per-tile blocks_sight flag in atlas metadata; default true for walls
    local up_gid = M.cell_gid("upper_wall", x, y, id)
    if up_gid ~= 0 then
        local atlas_idx, tile_id, _rot = decode_gid(up_gid)
        local atlas = m.atlases[atlas_idx + 1]
        local tile = atlas and atlas.tiles[tile_id]
        if tile and tile.blocks_sight == false then
            return false
        end
        return true
    end
    return false
end

function M.tilemap_id(map_id)
    local id = map_id or current_map_id
    local m = map_registry[id]
    -- v2: return first atlas alias (backwards-compat for single-atlas maps)
    if m.schema_version >= 2 then
        return m.atlas_aliases and m.atlas_aliases[1] or (m.atlases[1] and m.atlases[1].id)
    end
    return m.tilemap.id
end

function M.current()
    return current_map_id
end

function M.set_current(map_id)
    if not map_registry[map_id] then
        error(string.format("maps.set_current: unknown map-id '%s'", tostring(map_id)))
    end
    current_map_id = map_id
end

function M.list()
    local out = {}
    for id, _ in pairs(map_registry) do
        out[#out+1] = id
    end
    return out
end

function M.atlas_count(map_id)
    local id = map_id or current_map_id
    if not id then error("maps.atlas_count: no current map") end
    local m = map_registry[id]
    if m.schema_version < 2 then return 1 end  -- v1 (pre-upgrade) always 1
    return #m.atlases
end

function M.atlas_id_at(idx, map_id)
    local id = map_id or current_map_id
    if not id then error("maps.atlas_id_at: no current map") end
    local m = map_registry[id]
    return m.atlas_aliases and m.atlas_aliases[idx + 1]
        or m.atlases[idx + 1] and m.atlases[idx + 1].id
end

-- 0.5.4: Public accessors for palette-like consumers (map-editor) that
-- need to render real tile thumbnails. All three return nil when the
-- atlas-idx is out of range, the slot has no tile in the atlas, or the
-- texture failed to load (headless test env).
function M.atlas_diffuse_handle(atlas_idx, map_id)
    local id = map_id or current_map_id
    if not id then error("maps.atlas_diffuse_handle: no current map") end
    local m = map_registry[id]
    local atlas = m.atlases and m.atlases[atlas_idx + 1]
    return atlas and atlas.diffuse_texture_handle or nil
end

function M.atlas_tile_size_px(atlas_idx, map_id)
    local id = map_id or current_map_id
    if not id then error("maps.atlas_tile_size_px: no current map") end
    local m = map_registry[id]
    local atlas = m.atlases and m.atlases[atlas_idx + 1]
    return atlas and atlas.tile_size_px or nil
end

-- 0.5.5: public version of what the v3 vertex/material render path
-- computes internally. Returns {slot, rot, flip} for a material cell
-- (override entry takes precedence, else vertex-bitmask-derived), or
-- nil when the cell is non-material / the layer is non-material-bound.
-- Primary consumer: debug-overlay tooling that wants to label each
-- rendered cell with its blob-14 slot identity.
function M.cell_material_slot(layer_name, x, y, map_id)
    local id = map_id or current_map_id
    if not id then error("maps.cell_material_slot: no current map") end
    local m = map_registry[id]
    if not m.layers or not m.layers[layer_name] then return nil end
    local layer = m.layers[layer_name]
    if not layer.material then return nil end
    if x < 0 or x >= m.size.w or y < 0 or y >= m.size.h then return nil end
    -- Override wins (sparse force-slot map)
    local entry = layer.overrides and layer.overrides[x .. ":" .. y]
    if entry ~= nil then
        return normalize_override_entry(entry)
    end
    -- Vertex-derived bitmask path (only if the layer has a vertex grid)
    if not layer.vertices then return nil end
    if not cell_has_material(layer.vertices, layer.cells_material, m.size.w, m.size.h, x, y) then return nil end
    local bitmask = compute_cell_bitmask_v3(layer.vertices, layer.cells_material, m.size.w, m.size.h, x, y)
    return SLOT_LOOKUP[bitmask]
end

function M.atlas_tile_uv(atlas_idx, slot, map_id)
    local id = map_id or current_map_id
    if not id then error("maps.atlas_tile_uv: no current map") end
    local m = map_registry[id]
    local atlas = m.atlases and m.atlases[atlas_idx + 1]
    if not atlas or not atlas.tiles then return nil end
    local slot_idx = atlas_slot_index(atlas)
    local tile_id = slot_idx[slot]
    local tile = tile_id and atlas.tiles[tile_id]
    return tile and tile.uv or nil
end

function M.has_layer(layer_name, map_id)
    local id = map_id or current_map_id
    if not id then error("maps.has_layer: no current map") end
    local m = map_registry[id]
    return m.layers and m.layers[layer_name] ~= nil
end

function M.cell_gid(layer_name, x, y, map_id)
    local id = map_id or current_map_id
    if not id then error("maps.cell_gid: no current map") end
    local m = map_registry[id]
    if not m.layers or not m.layers[layer_name] then return 0 end
    if x < 0 or x >= m.size.w or y < 0 or y >= m.size.h then return 0 end
    local layer = m.layers[layer_name]
    -- v3 vertex-painted layer: return a sentinel non-zero for presence
    -- checks. callers that need the encoded GID (e.g. blocks_sight per-
    -- tile flag) get the safe default behaviour because decode_gid(1)
    -- yields a missing tile-record. Overrides (sparse force-slot map)
    -- also count as material-presence so is_walkable + blocks_walk +
    -- blocks_sight treat override-placed cells consistently with
    -- vertex-derived cells.
    if layer.vertices and not layer.tiles then
        if layer.overrides and layer.overrides[x .. ":" .. y] ~= nil then
            return 1
        end
        return cell_has_material(layer.vertices, layer.cells_material, m.size.w, m.size.h, x, y) and 1 or 0
    end
    return layer.tiles and layer.tiles[y * m.size.w + x + 1] or 0
end

-- =====================================================================
-- Write APIs (v0.4.0)
-- =====================================================================

-- Helper: gather valid layer names for error messages.
local function get_valid_layer_names_list()
    local names = {}
    for name, _ in pairs(VALID_LAYER_NAMES) do
        table.insert(names, name)
    end
    table.sort(names)
    return names
end

-- Internal: get-or-allocate a layer table for in-place writes.
local function get_or_alloc_layer(map, layer_name)
    if not VALID_LAYER_NAMES[layer_name] then
        error(string.format("maps: unknown layer '%s' (valid: %s)",
            layer_name, table.concat(get_valid_layer_names_list(), ", ")))
    end
    local layer = map.layers[layer_name]
    if not layer then
        local cell_count = map.size.w * map.size.h
        local tiles = {}
        for i = 1, cell_count do tiles[i] = 0 end
        layer = { tiles = tiles }
        map.layers[layer_name] = layer
    end
    return layer
end

function M.set_cell_gid(layer_name, x, y, gid, map_id)
    local map = require_map(map_id)
    if x < 0 or y < 0 or x >= map.size.w or y >= map.size.h then
        error(string.format("maps.set_cell_gid: cell (%d,%d) out of bounds for size %dx%d",
            x, y, map.size.w, map.size.h))
    end
    local layer = get_or_alloc_layer(map, layer_name)
    local idx = y * map.size.w + x + 1
    layer.tiles[idx] = gid
    map._dirty = true
    -- 0.5.0e: incremental layer-content cache invalidation.
    -- Writes that ADD content (gid != 0) mark the layer present;
    -- writes that clear (gid == 0) leave the cache as-is — the worst
    -- case is one extra iteration of an empty layer, harmless.
    if gid ~= 0 and map._layer_has_content then
        map._layer_has_content[layer_name] = true
    end
    -- Opaque-ceiling cache also depends on this cell; full nil-out and
    -- lazy rebuild on next render is simplest + correct.
    map._opaque_ceiling = nil
end

-- v3 vertex-painted-layer override APIs.
-- 0.5.0d: override forces a specific canonical slot (0..13) at a cell,
--         bypassing the bitmask autotile result. Override implies
--         cell-has-material regardless of the vertex-grid state.
-- 0.5.1: override entry can be bare integer slot_id (canonical orientation,
--         compact-form, backwards-compat) OR object {slot, rot?, flip?}
--         for explicit-transform direct-mode placement.
-- Stored as sparse map keyed "x:y" -> entry.

local function require_v3_layer(map, layer_name, fn_name)
    if map.schema_version < 3 then
        error(string.format("%s: schema-v3 only; current map is v%d",
            fn_name, map.schema_version))
    end
    if not VALID_LAYER_NAMES[layer_name] then
        error(string.format("%s: unknown layer '%s' (valid: %s)",
            fn_name, layer_name, table.concat(get_valid_layer_names_list(), ", ")))
    end
    local layer = map.layers[layer_name]
    if not layer then
        error(string.format("%s: layer '%s' not present on map '%s'",
            fn_name, layer_name, map.id))
    end
    return layer
end

local function check_coords(map, x, y, fn_name)
    if x < 0 or y < 0 or x >= map.size.w or y >= map.size.h then
        error(string.format("%s: cell (%d,%d) out of bounds for size %dx%d",
            fn_name, x, y, map.size.w, map.size.h))
    end
end

-- 0.5.1: accepts either bare slot_id (compact form) or table
-- {slot, rot?, flip?} (explicit transform). Auto-stores as compact form
-- when rot+flip are both 0, else stores as object — minimises disk diff
-- for the common canonical-orientation case.
function M.set_override(layer_name, x, y, slot_or_entry, map_id)
    local map = require_map(map_id)
    check_coords(map, x, y, "maps.set_override")
    local layer = require_v3_layer(map, layer_name, "maps.set_override")
    local stored
    if type(slot_or_entry) == "number" then
        if slot_or_entry < 0 or slot_or_entry > 13
           or slot_or_entry ~= math.floor(slot_or_entry) then
            error(string.format("maps.set_override: slot %s must be integer 0..13",
                tostring(slot_or_entry)))
        end
        stored = slot_or_entry
    elseif type(slot_or_entry) == "table" then
        local slot = slot_or_entry.slot
        local rot  = slot_or_entry.rot  or 0
        local flip = slot_or_entry.flip or 0
        if type(slot) ~= "number" or slot < 0 or slot > 13
           or slot ~= math.floor(slot) then
            error(string.format("maps.set_override: object.slot %s must be integer 0..13",
                tostring(slot)))
        end
        if type(rot) ~= "number" or rot < 0 or rot > 3 or rot ~= math.floor(rot) then
            error(string.format("maps.set_override: object.rot %s must be integer 0..3",
                tostring(rot)))
        end
        if flip ~= 0 and flip ~= 1 then
            error(string.format("maps.set_override: object.flip %s must be 0 or 1",
                tostring(flip)))
        end
        if rot == 0 and flip == 0 then
            stored = slot  -- canonical orientation -> compact form
        else
            stored = { slot = slot, rot = rot, flip = flip }
        end
    else
        error(string.format("maps.set_override: must be integer slot or {slot, rot?, flip?} object, got %s",
            type(slot_or_entry)))
    end
    if layer.overrides == nil then layer.overrides = {} end
    layer.overrides[x .. ":" .. y] = stored
    map._dirty = true
    if map._layer_has_content then
        map._layer_has_content[layer_name] = true
    end
    map._opaque_ceiling = nil
end

function M.clear_override(layer_name, x, y, map_id)
    local map = require_map(map_id)
    check_coords(map, x, y, "maps.clear_override")
    local layer = require_v3_layer(map, layer_name, "maps.clear_override")
    if layer.overrides then
        layer.overrides[x .. ":" .. y] = nil
        map._dirty = true
        map._opaque_ceiling = nil
    end
end

-- 0.5.1: returns normalized form {slot, rot, flip} regardless of how
-- the entry is stored on disk (bare-int compact OR object form). Returns
-- nil when no override is set. Callers that need the on-disk shape
-- should consult layer.overrides directly.
function M.get_override(layer_name, x, y, map_id)
    local map = require_map(map_id)
    check_coords(map, x, y, "maps.get_override")
    local layer = require_v3_layer(map, layer_name, "maps.get_override")
    if layer.overrides == nil then return nil end
    local entry = layer.overrides[x .. ":" .. y]
    if entry == nil then return nil end
    return normalize_override_entry(entry)
end

-- 0.5.2: vertex-grid write APIs for the autotile painting path.
-- Vertices live on a (W+1) x (H+1) grid; painting a single vertex
-- "fills in" the 4 cells around it per the any-corner rule (cell
-- becomes material). The map-editor's Auto-Tile mode is the
-- primary consumer.

local function check_vertex_coords(map, vx, vy, fn_name)
    local vw = map.size.w + 1
    local vh = map.size.h + 1
    if vx < 0 or vy < 0 or vx >= vw or vy >= vh then
        error(string.format("%s: vertex (%d,%d) out of bounds for vertex-grid %dx%d",
            fn_name, vx, vy, vw, vh))
    end
end

local function ensure_vertex_grid(layer, map)
    if layer.vertices == nil then
        local vw = map.size.w + 1
        local vh = map.size.h + 1
        local grid = {}
        for i = 1, vw * vh do grid[i] = 0 end
        layer.vertices = grid
    end
    return layer.vertices
end

function M.set_vertex(layer_name, vx, vy, painted, map_id)
    local map = require_map(map_id)
    check_vertex_coords(map, vx, vy, "maps.set_vertex")
    local layer = require_v3_layer(map, layer_name, "maps.set_vertex")
    local v = ensure_vertex_grid(layer, map)
    local vw = map.size.w + 1
    local idx = vy * vw + vx + 1
    local new_val = (painted == true or painted == 1) and 1 or 0
    if v[idx] ~= new_val then
        v[idx] = new_val
        map._dirty = true
        map._opaque_ceiling = nil  -- vertex change may affect opaque ceiling
        -- _layer_has_content goes true on paint; never auto-cleared
        -- on erase (stale-true is harmless).
        if new_val == 1 and map._layer_has_content then
            map._layer_has_content[layer_name] = true
        end
    end
end

function M.get_vertex(layer_name, vx, vy, map_id)
    local map = require_map(map_id)
    check_vertex_coords(map, vx, vy, "maps.get_vertex")
    local layer = require_v3_layer(map, layer_name, "maps.get_vertex")
    if layer.vertices == nil then return false end
    local vw = map.size.w + 1
    local v = layer.vertices[vy * vw + vx + 1]
    return v == 1 or v == true
end

-- 0.5.7: set/clear the per-cell material flag at (cx, cy) on the named
-- layer. painted: bool. Lazy-allocates layer.cells_material on first
-- write. Out-of-bounds raises a bounds error. Unknown layer raises a
-- layer-not-found error.
function M.set_cell_material(layer_name, cx, cy, painted, map_id)
    local m = require_map(map_id)
    local layer = m.layers[layer_name]
    if not layer then
        error(string.format("maps.set_cell_material: no layer '%s'", layer_name))
    end
    local w, h = m.size.w, m.size.h
    if cx < 0 or cx >= w or cy < 0 or cy >= h then
        error(string.format("maps.set_cell_material: out of bounds (%d, %d) for %dx%d map",
            cx, cy, w, h))
    end
    local cm = ensure_cells_material(layer, w, h)
    cm[cy * w + cx + 1] = painted and 1 or 0
    m._dirty = true
    m._opaque_ceiling = nil
end

-- 0.5.7: read the per-cell material flag. Returns false if
-- layer.cells_material is absent or the entry is unset. Does NOT
-- include vertex-derived material — call cell_material_slot or render
-- if you need the effective state.
function M.get_cell_material(layer_name, cx, cy, map_id)
    local m = require_map(map_id)
    local layer = m.layers[layer_name]
    if not layer then
        error(string.format("maps.get_cell_material: no layer '%s'", layer_name))
    end
    local w, h = m.size.w, m.size.h
    if cx < 0 or cx >= w or cy < 0 or cy >= h then return false end
    if layer.cells_material == nil then return false end
    local v = layer.cells_material[cy * w + cx + 1]
    return v ~= nil and v ~= 0 and v ~= false
end

function M.set_roof(x, y, value, map_id)
    local map = require_map(map_id)
    if x < 0 or y < 0 or x >= map.size.w or y >= map.size.h then
        error(string.format("maps.set_roof: cell (%d,%d) out of bounds for size %dx%d",
            x, y, map.size.w, map.size.h))
    end
    if value ~= 0 and value ~= 1 then
        error(string.format("maps.set_roof: value %s must be 0 or 1", tostring(value)))
    end
    if not map.roof then
        local cell_count = map.size.w * map.size.h
        map.roof = {}
        for i = 1, cell_count do map.roof[i] = 0 end
    end
    local idx = y * map.size.w + x + 1
    map.roof[idx] = value
    map._dirty = true
end

-- =====================================================================
-- Internal JSON pretty-printer (hand-rolled, 2-space indent).
-- Supports: nil/null, bool, number (int + float), string, array, object.
-- Deterministic key ordering (sorted) so saves are byte-stable.
-- =====================================================================
local json_value  -- forward declaration
local json_table  -- forward declaration

local function json_escape_string(s)
    local replacements = {
        ['"']  = '\\"',
        ['\\'] = '\\\\',
        ['\n'] = '\\n',
        ['\r'] = '\\r',
        ['\t'] = '\\t',
        ['\b'] = '\\b',
        ['\f'] = '\\f',
    }
    return '"' .. s:gsub('[%z\1-\31"\\]', function(c)
        return replacements[c] or string.format('\\u%04x', string.byte(c))
    end) .. '"'
end

json_value = function(v, indent)
    if v == nil then return "null" end
    local t = type(v)
    if t == "boolean" then return v and "true" or "false" end
    if t == "number" then
        if v ~= v then return "null" end                       -- NaN
        if v == math.huge or v == -math.huge then return "null" end
        if math.type(v) == "integer" or v == math.floor(v) then
            return string.format("%d", v)
        end
        return string.format("%.17g", v)
    end
    if t == "string" then return json_escape_string(v) end
    if t == "table" then
        return json_table(v, indent)
    end
    error("json: cannot serialize value of type " .. t)
end

json_table = function(t, indent)
    -- Detect array vs object by checking integer keys 1..n
    local n = #t
    local is_array = n > 0
    if is_array then
        for k, _ in pairs(t) do
            if type(k) ~= "number" then is_array = false; break end
        end
    end
    local indent_next = indent .. "  "

    if is_array then
        if n == 0 then return "[]" end
        local parts = {}
        for i = 1, n do
            parts[i] = indent_next .. json_value(t[i], indent_next)
        end
        return "[\n" .. table.concat(parts, ",\n") .. "\n" .. indent .. "]"
    end

    -- Object: collect & sort keys for determinism
    local keys = {}
    for k, _ in pairs(t) do
        table.insert(keys, tostring(k))
    end
    table.sort(keys)
    if #keys == 0 then return "{}" end
    local parts = {}
    for _, k in ipairs(keys) do
        parts[#parts + 1] = indent_next .. json_escape_string(k) ..
            ": " .. json_value(t[k], indent_next)
    end
    return "{\n" .. table.concat(parts, ",\n") .. "\n" .. indent .. "}"
end

-- =====================================================================
-- v2-Map serialization: reverse the internal resolved shape back to the
-- on-disk JSON schema (atlases as ID strings, layers in canonical order).
-- =====================================================================
local LAYER_DISK_ORDER = {
    "foundation", "subsurface", "surface", "topsurface",
    "lower_wall", "wall", "upper_wall", "canopy",
}

local function serialize_map_v3(map)
    local out = {
        schema_version = 3,
        id   = map.id,
        size = { w = map.size.w, h = map.size.h },
        atlases = {},
        layers = {},
    }
    -- Use atlas_aliases (raw string IDs from disk) for round-trip fidelity.
    for i, alias in ipairs(map.atlas_aliases) do
        out.atlases[i] = alias
    end
    for _, layer_name in ipairs(LAYER_DISK_ORDER) do
        local layer = map.layers[layer_name]
        if layer then
            local layer_out = {}
            if layer.tiles then
                local tiles_copy = {}
                for i = 1, #layer.tiles do tiles_copy[i] = layer.tiles[i] end
                layer_out.tiles = tiles_copy
            end
            -- Preserve forward-compat fields if the loaded map had them set.
            if layer.material  ~= nil then layer_out.material  = layer.material  end
            if layer.vertices  ~= nil then layer_out.vertices  = layer.vertices  end
            if layer.overrides ~= nil then layer_out.overrides = layer.overrides end
            out.layers[layer_name] = layer_out
        end
    end
    if map.roof then
        local roof_copy = {}
        for i = 1, #map.roof do roof_copy[i] = map.roof[i] end
        out.roof = roof_copy
    end
    return json_value(out, "") .. "\n"
end

function M.save_to_disk(map_id, path)
    local map = require_map(map_id)
    local json_str = serialize_map_v3(map)
    local f, err = io.open(path, "w")
    if not f then
        error(string.format("maps.save_to_disk: cannot open '%s' (%s)",
            path, err or "?"))
    end
    f:write(json_str)
    f:close()
    map._dirty = false
end

function M.is_indoor(x, y, map_id)
    local id = map_id or current_map_id
    if not id then error("maps.is_indoor: no current map") end
    local m = map_registry[id]
    if not m.roof then return false end
    if x < 0 or x >= m.size.w or y < 0 or y >= m.size.h then return false end
    return m.roof[y * m.size.w + x + 1] == 1
end

-- =====================================================================
-- Atlas-Loading (M.2: paired-PNG diffuse + height atlases)
-- Replaces the M.1-era tilemap-indexed loader. Each atlas-set lives at
-- <asset-lib>/assets/atlases/<atlas_id>/ with four files:
--   tiles.diffuse.atlas.png  — RGBA8888 packed atlas
--   tiles.height.atlas.png   — L8 grayscale heightmap
--   tiles.atlas.json         — metadata + per-tile UV-rects
--   tiles.atlas.lock.json    — name -> id stable bindings (not loaded
--                              at runtime; only used by the baker)
-- =====================================================================
function M.load_textures(asset_aliases)
    local map_id = M.current()
    if map_id == nil then
        error("maps.load_textures: no current map; call maps.set_current first")
    end
    local m = map_registry[map_id]
    if m.schema_version < 2 then
        error("maps.load_textures: only schema-v2 maps supported in v0.3.0+")
    end

    for atlas_idx, atlas_alias in ipairs(m.atlas_aliases) do
        local lib_id = asset_aliases[atlas_alias]
        if lib_id == nil then
            error(string.format(
                "maps.load_textures: asset-alias '%s' not in module asset_aliases",
                atlas_alias))
        end
        local base = lib_id .. "/assets/atlases/" .. atlas_alias
        local meta = engine.asset.load_json(base .. "/tiles.atlas.json")
        if meta.atlas_id ~= atlas_alias then
            error(string.format(
                "maps.load_textures: atlas_id mismatch in %s (json has '%s')",
                base, meta.atlas_id))
        end
        -- Build tile-record dictionary keyed by integer ID
        local tiles_by_id = {}
        for _, t in ipairs(meta.tiles) do
            tiles_by_id[t.id] = {
                id           = t.id,
                name         = t.name,
                walkable     = (t.walkable == true),
                uv           = { x = t.uv[1], y = t.uv[2], w = t.uv[3], h = t.uv[4] },
                blocks_sight = t.blocks_sight,
            }
        end
        -- Texture loading may fail in headless test environments (no OpenGL
        -- context). Use pcall so that tile-record shape + walkability tests
        -- still pass; only the handle is nil when loading is unavailable.
        local ok_d, diffuse_h = pcall(engine.asset.load_texture, base .. "/tiles.diffuse.atlas.png")
        local ok_h, height_h  = pcall(engine.asset.load_texture, base .. "/tiles.height.atlas.png")
        m.atlases[atlas_idx] = {
            id = atlas_alias,
            atlas_id = meta.atlas_id,
            atlas_size_px = meta.atlas_size_px,
            tile_size_px = meta.tile_size_px,
            tiles = tiles_by_id,
            diffuse_texture_handle = ok_d and diffuse_h or nil,
            height_texture_handle  = ok_h and height_h  or nil,
        }
        -- v0.5.3: keep atlas_by_alias in sync with the replaced atlas
        -- record. Built at load time off the pre-load stub; without this
        -- refresh the v3 vertex/material render path resolves through
        -- atlas_by_alias to the stub and falls back to MISSING_ASSET_COLOR.
        if m.atlas_by_alias then
            m.atlas_by_alias[atlas_alias] = m.atlases[atlas_idx]
        end
    end

    -- Update m.tile_size from first atlas only when the atlas declares a uniform
    -- tile size. Variable-size atlases (tile_size_px = null) keep the tile_size
    -- that was set by the stub or tilemap JSON at load time (typically 32).
    if m.atlases[1] and m.atlases[1].tile_size_px ~= nil then
        m.tile_size = m.atlases[1].tile_size_px
    end
end

-- Returns the diffuse atlas-texture-handle for a layer, IF all cells in
-- the layer reference the same atlas-index. For multi-atlas layers
-- (cells with mixed atlas_idx in their packed-u32 GIDs) returns nil —
-- consumers must fall back to per-cell sampling. (M.2)
function M.get_layer_diffuse_texture(layer_name, map_id)
    local id = map_id or current_map_id
    if not id then error("maps.get_layer_diffuse_texture: no current map") end
    local m = map_registry[id]
    if not m.layers or not m.layers[layer_name] then return nil end
    local tiles = m.layers[layer_name].tiles
    local atlas_idx = nil
    for i = 1, #tiles do
        local gid = tiles[i] or 0
        if gid ~= 0 then
            local a = (gid >> 24) & 0xFF
            if atlas_idx == nil then
                atlas_idx = a
            elseif atlas_idx ~= a then
                return nil  -- mixed atlases
            end
        end
    end
    if atlas_idx == nil then return nil end  -- empty layer
    local atlas = m.atlases[atlas_idx + 1]
    return atlas and atlas.diffuse_texture_handle or nil
end

-- Same as above but for the height-channel. (M.2)
function M.get_layer_height_texture(layer_name, map_id)
    local id = map_id or current_map_id
    if not id then error("maps.get_layer_height_texture: no current map") end
    local m = map_registry[id]
    if not m.layers or not m.layers[layer_name] then return nil end
    local tiles = m.layers[layer_name].tiles
    local atlas_idx = nil
    for i = 1, #tiles do
        local gid = tiles[i] or 0
        if gid ~= 0 then
            local a = (gid >> 24) & 0xFF
            if atlas_idx == nil then
                atlas_idx = a
            elseif atlas_idx ~= a then
                return nil
            end
        end
    end
    if atlas_idx == nil then return nil end
    local atlas = m.atlases[atlas_idx + 1]
    return atlas and atlas.height_texture_handle or nil
end

function M.iterate_layers_pre_entities(fn, map_id)
    local id = map_id or current_map_id
    if not id then error("maps.iterate_layers_pre_entities: no current map") end
    local m = map_registry[id]
    if m.schema_version < 2 then
        -- v1 (legacy): only surface conceptually
        fn("surface")
        return
    end
    for _, name in ipairs(LAYER_ORDER_PRE_ENTITIES) do
        if m.layers[name] then fn(name) end
    end
end

function M.iterate_layers_post_entities(fn, map_id)
    local id = map_id or current_map_id
    if not id then error("maps.iterate_layers_post_entities: no current map") end
    local m = map_registry[id]
    if m.schema_version < 2 then return end  -- v1 has nothing post
    for _, name in ipairs(LAYER_ORDER_POST_ENTITIES) do
        if m.layers[name] then fn(name) end
    end
end

function M.draw_map_pre_entities()
    local id = current_map_id
    if id == nil then return end
    local m = map_registry[id]
    if m.schema_version < 2 then
        draw_v1_legacy(m)
        return
    end
    local has = m._layer_has_content
    for _, name in ipairs(LAYER_ORDER_PRE_ENTITIES) do
        if m.layers[name] and (has == nil or has[name]) then
            draw_layer(m, name)
        end
    end
end

function M.draw_map_post_entities()
    local id = current_map_id
    if id == nil then return end
    local m = map_registry[id]
    if m.schema_version < 2 then return end
    local has = m._layer_has_content
    for _, name in ipairs(LAYER_ORDER_POST_ENTITIES) do
        if m.layers[name] and (has == nil or has[name]) then
            draw_layer(m, name)
        end
    end
end

-- Backward-compat: draw_map renders everything (pre + post, no entity slot)
function M.draw_map()
    M.draw_map_pre_entities()
    M.draw_map_post_entities()
end

-- Forward-compat stubs (DEPRECATED-MVP — implemented in later slices)
function M.state(map_id)
    -- DEPRECATED-MVP: lifecycle states (Virgin/Inert/Passive/Active/Pinned) — multi-map slice
    return "Active"
end

function M.pin(map_id, reason)
    -- DEPRECATED-MVP: world.pin_map mechanic — multi-map slice
    engine.warn("maps.pin: deferred to map-topology lifecycle slice")
end

M.encode_gid = encode_gid
M.decode_gid = decode_gid
M.upgrade_v1_to_v2 = upgrade_v1_to_v2
M.upgrade_v2_to_v3 = upgrade_v2_to_v3
M.validate_map_table_v2 = validate_map_table_v2
M.validate_map_table_v3 = validate_map_table_v3

return M