Persistent islands - islands are merged and split incrementally rather than being rebuilt every time step.
Sleeping is now per island rather than per body.

Merging uses union-find. Each body created also creates an island. When joints are created the islands are linked and merged. When contacts begin touching the islands are linked and merged.

When a contact stops touching or is destroyed the island is flagged for splitting. Likewise destroying a joint flags the island for splitting. Only a single island may be split per time step. The largest island is chosen. Island splitting is handled in the island job.

The parallel collide tasks generate state changes for contacts. These are processed in deterministic order to maintain overall determinism.