Run deferred BVH work if it is still queued

[dajiaohuang]

Summary

• Keep a deferred BVH optimization task available until a Rayon worker claims it.
• If the joining worker finds the task still queued, run it there; otherwise wait for the worker that claimed it.
• Add a deterministic two-worker regression test that parks the other worker and checks the pending-work handoff.

This addresses the multi-worker hang reported in #1008. The report describes an intermittent failure and does not establish a rate for any worker count.

Validation

• cargo test -p rapier3d --release --features parallel --lib deferred_bvh_join_runs_pending_work_when_other_worker_is_blocked (passed)
• cargo test -p rapier3d --release --features parallel --test single_worker_deferred_bvh (passed)
• cargo fmt --check (passed)
• git diff --check (passed)