Render magenta placeholder when atlas is unloaded

When a consumer renders via maps.draw_map_pre/post_entities without
having called maps.load_textures (or when asset-alias resolution
failed), the per-atlas diffuse_texture_handle is nil. The previous
draw_layer passed nil to draw_sprite_transform and crashed. Now it
falls back to engine.render.draw_rect with the classic magenta
missing-asset color so the consumer sees the gap visually instead.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

init.lua

@@ -339,6 +339,9 @@ end
 
 -- 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
@@ -351,11 +354,11 @@ local function draw_layer(m, layer_name)
             if gid ~= 0 then
                 local atlas_idx, tile_id, rot_quad = decode_gid(gid)
                 local atlas = m.atlases[atlas_idx + 1]
-                if atlas then
+                local px = x * ts
+                local py = y * ts
+                if atlas and atlas.diffuse_texture_handle then
                     local tile = atlas.tiles[tile_id]
-                    if tile then
+                    if tile and tile.uv then
-                        local px = x * ts
-                        local py = y * ts
                         local rot_deg = rot_quad * 90.0
                         engine.render.draw_sprite_transform(
                             atlas.diffuse_texture_handle,
@@ -366,7 +369,15 @@ local function draw_layer(m, layer_name)
                             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
         end