Restore mutex-protected worker shutdown tracking

[simsong-codex]

Summary

• restore the live-worker set as the authoritative shutdown predicate
• protect worker registration, removal, observation, and join predicates with the pool mutex
• retain explicit std::thread::join() calls and make worker/thread registration rollback-safe
• add 25 timed shutdown attempts with 32 idle workers per attempt
• document the user-visible shutdown fix in the 2.2.0 release notes
• prepare this open PR head as the v2.2.0beta2 release candidate tracked in release: v2.2.0beta2 #680

Root cause

The atomic workers_running counter was changed and followed by a condition-variable notification without holding the mutex used by thread_pool::join(). With multiple workers exiting, the last notification could occur after the join predicate was checked but before the joining thread actually slept. The counter could then be zero with no future notifier, leaving the timed join asleep until its deadline.

The restored worker set closes that window because worker removal and workers.empty() are ordered by the same mutex used by TO_MAIN.

Addresses #678.

Validation

• make -C src check TESTS=test_be20_api — pass
• make check — pass (4/4 test programs; 0 failures, 0 errors)
• make distcheck — pass (clean archive build, install/uninstall, and 4/4 test programs)
• GitHub Actions — pass on current head cbc2cc994d9b47330e27b582947e9a40d82c738b, including macOS, Ubuntu, AddressSanitizer, Windows runtime, MinGW, Codecov, documentation, Snap, and CodeQL
• Current-head Copilot review — pass on cbc2cc994d9b47330e27b582947e9a40d82c738b; 5/5 changed files reviewed with no new comments and no review threads

Beta test release

The signed, reviewed open-PR head is published as
v2.2.0beta2.
PR #679 remains open and unmerged while beta feedback is collected.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.79%. Comparing base (586a75a) to head (cbc2cc9).

Files with missing lines	Patch %	Lines
src/be20_api/threadpool.cpp	77.78%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #679      +/-   ##
==========================================
+ Coverage   77.78%   77.79%   +0.02%     
==========================================
  Files         118      118              
  Lines       14332    14361      +29     
==========================================
+ Hits        11147    11172      +25     
- Misses       3185     3189       +4     

Flag	Coverage Δ
unittests	77.79% <88.89%> (+0.02%)	⬆️

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[Copilot]

Pull request overview

This PR fixes an intermittent thread_pool::join() stall by restoring a mutex-ordered shutdown predicate based on a live-worker set, eliminating a lost-wakeup window that could leave timed joins sleeping until their deadline even after all workers had exited (Issue #678).

Changes:

• Replace the atomic “workers running” counter with a mutex-protected live workers set and use workers.empty() as the shutdown/join predicate.
• Make worker/thread registration rollback-safe and keep explicit std::thread::join() behavior.
• Add a regression test that repeatedly exercises timed idle-worker shutdown, and document the user-visible fix in the 2.2.0 release notes.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/be20_api/threadpool.h	Introduces a mutex-protected live-worker set to serve as the authoritative shutdown predicate.
src/be20_api/threadpool.cpp	Implements mutex-ordered worker registration/removal and updates join predicates to use workers.empty().
src/be20_api/test_be20_threadpool.cpp	Adds a repeated timed-join regression test to detect the prior idle-worker shutdown stall.
doc/RELEASE_NOTES.md	Documents the shutdown/join stall fix as a user-visible change for 2.2.0.

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[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated no new comments.