Improve frailbox logger error handling (#20)

[Soengkit]

Summary

Improves frailbox legacy logger error handling for issue #20.

/claim #20

Changes

• Tracks the configured LOG_FILE path for clearer diagnostics.
• Falls back to stderr when the configured log file cannot be opened, written, or flushed.
• Preserves the current log message by retrying it on stderr after a file write failure.
• Adds a small C logger error-handling harness and make test-logger-error target.
• Documents the stderr fallback behavior in operations troubleshooting.
• Keeps frailbox validation portable on non-Linux hosts while preserving Linux hardening flags and Linux-specific behavior.
• Fixes two build diagnostic hygiene issues: Python 3.9 parsing for build.py and password-like stdout being stored as an encryptly error.

Testing

• python3 -m py_compile build.py - PASS.
• make -C frailbox test-logger-error - PASS. The open-failure fallback path is verified; /dev/full write-failure coverage is skipped on macOS because /dev/full is unavailable.
• make -C frailbox - PASS.
• python3 build.py --module frailbox - PASS and generated committed metadata at diagnostic/build-81f3e49f.json.
• git diff --check - PASS.

Note: encryptly pack on this macOS runner exits 1 after writing 20 bytes to stdout and does not create diagnostic/build-81f3e49f.logd; the committed JSON records the frailbox PASS result and the .logd creation failure.

Checklist

• Relevant modules affected by these changes build locally
• Tests pass locally
• Diagnostic build log is committed in this PR (encryptly pack did not create a .logd; metadata is committed)
• Documentation has been updated, if applicable
• Configuration or schema changes are documented, if applicable
• No generated build artifacts are committed, except the required diagnostic metadata
• Changes are scoped to the PR purpose and avoid unrelated cleanup
• Security, privacy, and error-handling implications have been considered

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• I would like to request that my diagnostic build log is removed before merging