feat: stage 2 collision, TileChangedEvent, path invalidation, rabbit digging

- Stage 2 collision: convoy skip when entities move same direction (dot>0.7),
  E/S yield rule for crossings, W/N right-of-way, head-on unchanged
- TileChangedEvent (Message) + PathfindingDirtyChunks (Resource) in new tile_changed module
- collect_pathfinding_dirty_chunks / invalidate_paths_on_tile_change /
  clear_pathfinding_dirty_chunks in PathfindingPlugin FixedUpdate chain
- TileMap::remove_floor clears HashMap + ChunkData bitsets + tile_ids
- RabbitDigTimer component: rabbits dig floor below every 5s, fires TileChangedEvent
  and TileOcclusionEvent for path invalidation + rendering

src/world/mod.rs

@@ -48,6 +48,7 @@ impl Plugin for WorldPlugin {
             .add_message::<ChunkFoliageEvent>()
             .add_message::<ChunkFaunaEvent>()
             .add_message::<TileOcclusionEvent>()
+            .add_message::<tiles::TileChangedEvent>()
             .add_systems(Startup, (setup_chunk_system, setup_initial_chunks).chain())
             .add_systems(
                 FixedUpdate,