Prevent parser deadlocks and add Windows portability

[PhillipsT-Ai2]

Summary

• remove the queue-capacity deadlock and make parser shutdown cancellable
• add portable RSS, ownership, and fsync helpers
• use platform-appropriate storage paths and security checks
• add Windows CI coverage and stable line-ending rules

Verification

• 145 focused portable-core tests passed
• targeted mypy passed
• Ruff and staged diff checks passed

Known limitation

The legacy Unix-specific one-shot benchmark harness still depends on facilities such as fork, wait4, POSIX signals, and symlink privileges, so that harness is not fully runnable on Windows. The portable production paths and new Windows CI scope are covered.

[PhillipsT-Ai2]

Maintainer action requested: approve and start the fork pull-request workflows.

GitHub currently reports this PR as BLOCKED, and no pull-request workflow run or check suite has been created for the head SHA. The branch is conflict-free. Local validation completed successfully: 145 focused portable-core tests, targeted mypy, Ruff, and diff checks.

Could a maintainer please approve the Actions run and review the PR?