Wrap TileMap HashMaps in Arc for async pathfinding access

- Add Arc<AHashMap> wrapper around floor_tiles, fixture_tiles, item_tiles
- Copy-on-write semantics: Arc::make_mut clones only if other Arcs exist
- Add insert_floor, insert_fixture, insert_item, remove_item methods
- Add get_floor_mut, get_fixture_mut for visibility updates
- Update all mutation sites to use new TileMap methods
- Enables cheap Arc::clone for async pathfinding workers
- Single-threaded pathfinding: no clone, direct access
- Multi-threaded pathfinding: clone Arc, read without locks

src/world/generation/terrain.rs

@@ -159,7 +159,7 @@ pub fn generate_chunk_terrain(
         .unwrap()
         .into_iter()
         .map(|(pos, data)| {
-            tilemap.floor_tiles.insert(pos, data);
+            tilemap.insert_floor(pos, data);
             pos
         })
         .collect();