fix(sync): harden pre-Vista condition waiters

[jpnurmi]

Fix the pre-Vista condition-variable fallback so waiter accounting is incremented before the critical section is released, and waiter counts are read atomically before wake decisions.

The following review comments pointed out non-threadpool-related issues:

• cursor: "PREVISTA wake-all can deadlock"
  feat(internal): thread pool #1883 (comment)

• sentry: "non-atomic read of ConditionVariable->Waiters"
  feat(internal): thread pool #1883 (comment)

#skip-changelog

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.96%. Comparing base (58a511a) to head (614de79).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1960   +/-   ##
=======================================
  Coverage   75.95%   75.96%           
=======================================
  Files          94       94           
  Lines       22161    22161           
  Branches     3936     3936           
=======================================
+ Hits        16833    16835    +2     
+ Misses       4442     4439    -3     
- Partials      886      887    +1     

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