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)

src/world/generation/terrain.rs

@@ -8,9 +8,9 @@ use crate::{
     config::TileRegistry,
     constants::{SEED, TILE_SIZE},
     world::{
-        tiles::{ChunkData, FloorTileData, TileMap, TerrainSpriteState, CurrentWorldSpriteState},
+        tiles::{ChunkData, FloorTileData, TileMap},
         ChunkForrestryEvent, ChunkTerrainEvent, FloorTilePrefab, TileOcclusionEvent, CHUNK_SIZE,
-        Z_ABOVE, Z_BELOW, ChunkMap, ChunkOwner,
+        Z_ABOVE, Z_BELOW, ChunkMap,
     },
 };
 
@@ -265,7 +265,6 @@ fn generate_terrain_blob(chunk_pos: IVec2) -> TerrainBlob {
 /// Fast - just reads events and spawns tasks.
 pub fn spawn_terrain_tasks(
     mut events: MessageReader<ChunkTerrainEvent>,
-    mut cwss: ResMut<CurrentWorldSpriteState>,
     blob_storage: Res<TerrainBlobStorage>,
 ) {
     let start = Instant::now();
@@ -287,7 +286,6 @@ pub fn spawn_terrain_tasks(
         }).detach();
     }
 
-    cwss.state = TerrainSpriteState::WaitingForRender;
     println!("{} terrain tasks spawned in {:.2?}", count, start.elapsed());
 }
 
@@ -300,6 +298,7 @@ pub fn apply_terrain_blobs(
     mut chunk_map: ResMut<ChunkMap>,
     mut forrestry_event_writer: MessageWriter<ChunkForrestryEvent>,
     mut occlusion_event_writer: MessageWriter<TileOcclusionEvent>,
+    mut spawner: ResMut<crate::world::tiles::TilemapChunkSpawner>,
 ) {
     let start = Instant::now();
     let mut applied_count = 0;
@@ -317,16 +316,7 @@ pub fn apply_terrain_blobs(
             .collect();
 
         tilemap.chunks.insert(blob.chunk_pos, blob.chunk_data);
-
-        for (_position, prefab) in blob.tile_spawns {
-            let entity = prefab.spawn(&mut commands);
-            commands.entity(entity).insert(ChunkOwner(blob.chunk_pos));
-            chunk_map
-                .chunk_entity_index
-                .entry(blob.chunk_pos)
-                .or_default()
-                .push(entity);
-        }
+        spawner.queue_chunk(blob.chunk_pos);
 
         for pos in new_positions {
             occlusion_event_writer.write(TileOcclusionEvent { tile_position: pos });