Fix async thread-pool deadlock from semaphore-blocked B-team workers

[alexreinking]

A worker that found a job it could run but for an unavailable semaphore went to sleep on one of the two idle worker condition variables (the A/B teams), chosen by pool-size bookkeeping unrelated to why it slept. A semaphore release only ever broadcast wake_a_team, so a semaphore-blocked worker demoted to the B team was never woken by the release that made its job runnable, and the pipeline could deadlock with every thread parked.

The A/B teams model idle capacity (no runnable work), which is a different state from being blocked on an external event. Give blocked-on-semaphore workers their own wait channel and wake it on every release, alongside stalled owners. Genuinely-idle A/B-team workers are never waiting on a semaphore, so they are left undisturbed.

This also removes the old 0 -> 1-transition gate on the release wakeup, which was independently unsound for acquires of count > 1 satisfied by several count-1 releases in a row.

Adds test/correctness/async_deadlock.cpp, pinned to a low thread count (where the pool reshuffles teams and reliably exposes the bug).

Checklist

• Tests added or updated (not required for docs, CI config, or typo fixes)
• Documentation updated (if public API changed)
• Python bindings updated (if public API changed)
• Benchmarks are included here if the change is intended to affect performance.
• Commits include AI attribution where applicable (see Code of Conduct)

[abadams]

I think in future we could use a parallel deadlock fuzzer that tries random nestings of .parallel and .async and runs under tsan.

[mcourteaux]

The idea of the separate channel looks great, but I think there is still a race condition.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.06%. Comparing base (bb5426d) to head (168926f).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9372      +/-   ##
==========================================
- Coverage   70.08%   70.06%   -0.02%     
==========================================
  Files         259      260       +1     
  Lines       79158    79287     +129     
  Branches    19293    19327      +34     
==========================================
+ Hits        55477    55553      +76     
- Misses      17886    17916      +30     
- Partials     5795     5818      +23     

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