feat: data-oriented chunks optimization with tile registry

- Phase 1: Bit-packed standability (ChunkData with bitsets)
- Phase 2: Reactive connectivity (dirty chunks)
- Phase 3: Async terrain baking (AsyncComputeTaskPool)
- Pathfinding weight system (rock=50 preferred, bedrock=150 avoided)
- Movement speed affected by tile weight
- External tiles.toml for hot-reloadable tile definitions
- TileRegistry singleton for async-safe tile lookups
- Fixed world_to_chunk to use CHUNK_SIZE_TILE (128) not CHUNK_SIZE (8)
- Fixed infinite spawner with Local<bool> state guards
- Fixed spawn coordinate grid alignment

Note: Zigzag pathfinding bug introduced - needs investigation

src/game.rs

@@ -2,3 +2,6 @@ use bevy::prelude::*;
 
 #[derive(Resource)]
 pub struct ZIndex(pub f32);
+
+#[derive(Resource, Default)]
+pub struct SpawnDelay(pub u32);