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