Add set_cell_gid write API to lib-core.maps

Validates layer name against VALID_LAYER_NAMES, bounds-checks the
target cell, auto-allocates the layer table if the caller writes
to a previously-empty layer. Marks the map as dirty for callers
that track in-memory mutations.

init.lua

@@ -12,6 +12,15 @@ 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:
@@ -628,6 +637,49 @@ function M.cell_gid(layer_name, x, y, map_id)
     return m.layers[layer_name].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
+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