Class ExplosionManager

Batches all queued explosion events so that large simultaneous blasts are processed without blocking the main thread.

Key optimisations vs. the previous implementation:

1. Worker thread — character, vehicle, and construction damage math runs off the main thread; the main thread only applies the resulting plan.
2. Construction deduplication — nearby construction entities are collected once per chunk (not once per explosion), so the grid lookup cost is O(unique_positions) rather than O(chunk_size).
3. Character AABB pre-filter — a bounding box covering all explosion positions in the chunk prunes distant characters before sending to the worker, keeping the postMessage payload small.
4. setImmediate chunking — yields the event loop between CHUNK_SIZE batches so network I/O can interleave.
5. Deduplication — each entity is queued at most once per flush cycle.