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)

build.rs

@@ -0,0 +1,50 @@
+use image::{GenericImageView, ImageBuffer, Rgba};
+
+const TILE_PX: u32 = 16;
+
+const ROWS: &[(&str, u8)] = &[
+    ("assets/sky.png", 0),           // air (transparent)
+    ("assets/grass_floor.png", 1),   // grass
+    ("assets/dirt_floor.png", 2),    // dirt
+    ("assets/rock_floor.png", 3),    // rock
+    ("assets/bedrock_floor.png", 4), // bedrock
+    ("assets/sky.png", 5),           // sky (transparent)
+    ("assets/dirt_wall.png", 6),     // dirt_wall fixture
+    ("assets/rock_wall.png", 7),     // rock_wall fixture
+    ("assets/bedrock_wall.png", 8),  // bedrock_wall fixture
+    ("assets/log.png", 9),           // log fixture
+    ("assets/leaves.png", 10),       // leaves fixture
+];
+
+fn main() {
+    let rows = ROWS.len() as u32;
+    let mut combined: ImageBuffer<Rgba<u8>, Vec<u8>> =
+        ImageBuffer::from_pixel(TILE_PX, TILE_PX * rows, Rgba([0, 0, 0, 0]));
+
+    for (i, (path, _id)) in ROWS.iter().enumerate() {
+        let y = i as u32 * TILE_PX;
+        match image::open(path) {
+            Ok(img) => {
+                image::imageops::overlay(&mut combined, &img, 0, y as i64);
+                println!("cargo:rerun-if-changed={}", path);
+            }
+            Err(e) => {
+                eprintln!(
+                    "Warning: could not open {} — leaving row {} blank ({})",
+                    path, i, e
+                );
+                let mut row_pixels = combined.rows_mut();
+                if let Some(row) = row_pixels.nth(i) {
+                    for px in row {
+                        *px = Rgba([255, 0, 255, 255]);
+                    }
+                }
+            }
+        }
+    }
+
+    combined
+        .save("assets/tileset.png")
+        .expect("Failed to write assets/tileset.png");
+    println!("cargo:rerun-if-changed=build.rs");
+}