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/textures/management.rs

@@ -1,4 +1,7 @@
-use bevy::prelude::*;
+use bevy::{
+    image::{ImageArrayLayout, ImageLoaderSettings},
+    prelude::*,
+};
 use bevy_platform::collections::HashMap;
 use bevy_platform::time::Instant;
 
@@ -82,4 +85,17 @@ pub fn initialize_textures(mut commands: Commands, asset_server: Res<AssetServer
     commands.insert_resource(Textures { handles: textures });
     commands.insert_resource(TextureIDs { refs: texture_ids });
     println!("Textures initialized in {:.2?}", start.elapsed());
+
+    let tileset_handle =
+        asset_server.load_with_settings("tileset.png", |settings: &mut ImageLoaderSettings| {
+            settings.array_layout = Some(ImageArrayLayout::RowCount { rows: 11 });
+        });
+    commands.insert_resource(TilemapTileset {
+        handle: tileset_handle,
+    });
+}
+
+#[derive(Resource)]
+pub struct TilemapTileset {
+    pub handle: Handle<Image>,
 }