Fix Python 3.14 race in Windows contention test

[HLLMR]

Observed failure

Public PR #26 passed 14 of 15 required matrix jobs. windows-latest / Python 3.14 failed in test_windows_replacement_contention_retries_until_handle_releases.

The helper creates replace-contention-observed.txt before its write_text() call completes. The parent treats file existence as a complete signal and attempted int(''). That exception bypassed process.communicate(), leaving the helper process and privacy lock alive; temporary-directory cleanup then failed with WinError 32.

The product operation was not shown to fail. This is a deterministic test-harness publication/cleanup defect exposed by the required Python 3.14 leg.

Required correction

• Publish the contention observation atomically or wait for a complete, parseable signal.
• Guarantee child-process termination/reaping and handle release on every assertion/error path.
• Preserve the real Windows delete-sharing contention behavior and the accepted bounded retry contract.
• Exercise the affected test on the supported Windows matrix, including Python 3.14.
• Do not merge PR Fix Windows privacy writes and duplicate PR CI #26 until the complete required matrix passes.

Evidence

• Workflow run: https://github.com/HLLMR/writwall/actions/runs/33696519552
• Failed job: https://github.com/HLLMR/writwall/actions/runs/33696519552/job/100466442174
• PR: Fix Windows privacy writes and duplicate PR CI #26