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/main.rs

@@ -59,6 +59,7 @@ fn main() {
                 entities::sentient::dorf::spawn_dorfs,
                 entities::livestock::pig::spawn_pigs,
                 entities::livestock::rabbit::spawn_rabbits,
+                entities::livestock::rabbit::rabbit_dig_system,
             ),
         )
         .add_systems(Update, pig_drop_system)