fix: insert_floor syncs ChunkData, rabbit replaces dug tile with air

- TileMap::insert_floor now syncs ChunkData bits (set_floor_tile) so is_standable
  returns correct values for tiles inserted via insert_floor. Previously the
  HashMap was updated but ChunkData remained stale, causing is_standable to
  return false for valid tiles.
- rabbit_dig_system inserts air tile at the dug position after remove_floor.
  Without this, the HashMap entry is absent and ChunkData bits stay 0 (both
  false), making is_standable return false at that position. The entity then
  falls through multiple levels. Air's can_stand_in=true means the space is
  passable — the entity falls through it and lands on solid ground below.

src/world/tiles/tilemap.rs

@@ -190,6 +190,27 @@ impl TileMap {
 
     #[inline]
     pub fn insert_floor(&mut self, pos: IVec3, tile: FloorTileData) {
+        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);
+            if z >= -(Z_BELOW as i32)
+                && z <= Z_ABOVE as i32
+                && lx >= 0
+                && lx < CHUNK_SIZE
+                && ly >= 0
+                && ly < CHUNK_SIZE
+            {
+                chunk.set_floor_tile(
+                    lx,
+                    ly,
+                    z,
+                    tile.id,
+                    tile.can_stand_in(),
+                    tile.can_stand_on(),
+                    tile.astar_weight,
+                );
+            }
+        }
         self.floor_tiles.insert(pos, tile);
     }