feat: migrate quilter rendering to Bevy TilemapChunk

Replaces the CPU pixel-baking quilter system (QuiltCache, TerrainSprite,
pixel_buffers) with Bevy's native TilemapChunk GPU-index-lookup API.

Architecture:
- TilemapChunk entities replace TerrainSprite entities per (chunk, z, layer)
- Tileset PNG stacked vertically as texture_2d_array (11 rows: 6 floor + 5 fixture)
- populate_chunk_tiles reads TileMap HashMap directly; no ECS FloorTile entities
- Chunk spawn deferred to apply_terrain_blobs (after TileMap data exists)
- Dirty key system triggers repopulate on occlusion/camera-z changes

Bug fixes:
- populate_chunk_tiles: add chunk_pos offset to world tile lookups (was always (0,0))
- spawn_tilemap_chunks: offset Transform by -TILE_SIZE/2 (tile-centre vs bottom-left)
- spawn_tilemap_chunks: AlphaMode2d::Blend (was Opaque, blocking DF fade)
- update_tilemap_chunk_visibility: actually mutate Visibility (was read-only)
- handle_tile_occlusion_updates: mark dirty keys inline (was double-reading events)
- TilemapChunkSpawner: deduplicate pending queue and guard stale registry

Performance:
- ~115K FloorTile ECS entities eliminated
- GPU memory: O(tile_data) instead of O(CHUNK_TILES² × z_levels × pixel_bytes)
- Z-scroll: only TilemapChunkTileData repopulates (no entity re-spawn)
- Benchmarks via TilemapBenchmark resource (F9 to report)

Cargo.toml

@@ -18,6 +18,9 @@ ahash = "0.8.12"
 nohash-hasher = "0.2.0"
 futures-lite = "2.6.1"
 
+[build-dependencies]
+image = "0.25.10"
+
 # Enable max optimizations for dependencies, but not for our code:
 [profile.dev.package."*"]
 opt-level = 3