Windows/Linux test coverage for the shutdown, flush-at-exit and crash-flush paths #46

Several paths from #31, #34 and #35 are only exercised by CI builds, not by tests: the Windows console control handler (Ctrl+C / Break) termination flush, flush_at_exit on Windows (currently a no-op by design), the Linux/Android pipe2 path, and the SA_RESETHAND one-shot-handler case on Linux (macOS does not report that flag, so the test cannot prove the Linux behaviour). Add targeted tests, and run the shutdown suite on Windows where applicable.

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