Fix Windows GNU exports and improve coverage enforcement

[oferchen]

Summary

• wrap the Windows GNU exception helpers with #[unsafe(no_mangle)] to satisfy Rust 1.87's safety requirements
• add a reusable coverage enforcement script that prints summary metrics before comparing thresholds
• update the CI workflow to generate JSON and lcov coverage reports, then validate coverage via the new script

Testing

• cargo check -p rsync-windows-gnu-eh
• python3 tools/check_coverage.py /tmp/coverage-summary.json --min-lines 95 --min-functions 95

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https://chatgpt.com/codex/tasks/task_e_69035c94d724832395720bcd830fb075