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/tiles/tilemap.rs

@@ -198,6 +198,16 @@ impl TileMap {
         if let Some(chunk) = self.chunks.get_mut(&chunk_pos) {
             let (lx, ly, z) = ChunkData::world_to_local(pos);
             chunk.set_fixture_tile(lx, ly, z, tile.can_stand_in(), tile.can_stand_on());
+
+            // If fixture blocks entry, also clear the floor's stand_in bit at this position.
+            // Air floor tiles exist at above-ground positions — without this, air's
+            // can_stand_in=true would override the fixture block via the OR check.
+            if !tile.can_stand_in() {
+                let idx = ChunkData::pos_to_index(lx, ly, z);
+                let word = idx / 32;
+                let mask = !(1u32 << (idx % 32));
+                chunk.stand_in_floor[word] &= mask;
+            }
         }
         self.fixture_tiles.insert(pos, tile);
     }
@@ -260,6 +270,24 @@ impl TileMap {
         removed
     }
 
+    /// Remove a floor tile, clearing the HashMap entry, ChunkData bitsets, and tile_ids.
+    /// All three must be cleared — leaving ChunkData stale causes is_standable bugs,
+    /// and leaving tile_ids non-zero causes the renderer to keep drawing the tile.
+    pub fn remove_floor(&mut self, pos: &IVec3) -> Option<FloorTileData> {
+        let removed = self.floor_tiles.remove(pos);
+        let chunk_pos = world_to_chunk(*pos);
+        if let Some(chunk) = self.chunks.get_mut(&chunk_pos) {
+            let (lx, ly, z) = ChunkData::world_to_local(*pos);
+            let idx = ChunkData::pos_to_index(lx, ly, z);
+            let word = idx / 32;
+            let clear_mask = !(1u32 << (idx % 32));
+            chunk.stand_in_floor[word] &= clear_mask;
+            chunk.stand_on_floor[word] &= clear_mask;
+            chunk.tile_ids[idx] = 0;
+        }
+        removed
+    }
+
     /// Remove all tile data for a specific chunk from the TileMap.
     /// Iterates all positions in the chunk volume and removes from HashMaps.
     /// Used during chunk unloading to clean up tile data.