feat(optimization): implement data-oriented chunk architecture

Phase 1: Bit-packed standability
- Add ChunkData struct with 4 bitsets per chunk (stand_in/on for floor/fixture)
- Replace 4 HashMap lookups per standability check with O(1) bit operations
- Memory: ~2KB bitsets per chunk vs ~50KB HashMap overhead

Phase 2: Reactive connectivity
- Add dirty_chunks HashSet to ChunkMap for incremental updates
- update_chunk_connectivity now O(d) where d = dirty chunks
- Early exit when no changes, preventing O(N) full rebuilds

Phase 3: Async terrain baking
- Move terrain generation to AsyncComputeTaskPool
- spawn_terrain_tasks: non-blocking task spawn (~34µs)
- apply_terrain_blobs: batched entity spawn on main thread
- Eliminates main-thread stutters during world generation

src/entities/shared_systems/pathfinding.rs

@@ -550,30 +550,7 @@ fn validate_next_steps(tilemap: &TileMap, path: &[Vec3], start_index: usize, ste
 }
 
 fn is_standable_tile(tilemap: &TileMap, pos: IVec3) -> bool {
-    let can_stand_in_tile = tilemap
-        .floor_tiles
-        .get(&pos)
-        .map(|t| t.can_stand_in())
-        .unwrap_or(false);
-    let can_stand_in_fixture = tilemap
-        .fixture_tiles
-        .get(&pos)
-        .map(|t| t.can_stand_in())
-        .unwrap_or(false);
-    let pos_below = pos - IVec3::new(0, 0, ITILE_SIZE);
-    let can_stand_on_tile_below = tilemap
-        .floor_tiles
-        .get(&pos_below)
-        .map(|t| t.can_stand_on())
-        .unwrap_or(false);
-    let can_stand_on_fixture_below = tilemap
-        .fixture_tiles
-        .get(&pos_below)
-        .map(|t| t.can_stand_on())
-        .unwrap_or(false);
-
-    (can_stand_in_tile || can_stand_in_fixture)
-        && (can_stand_on_tile_below || can_stand_on_fixture_below)
+    tilemap.is_standable(pos)
 }
 
 fn calculate_movement_cost(move_dir: IVec3) -> i32 {