fix(frailbox): improve legacy logger file error handling (Closes #20)

[yanyishuai]

Summary

Improves legacy frailbox logger resilience when LOG_FILE cannot be opened or when write/flush operations fail. Logging continues on stderr with clearer diagnostics instead of failing silently.

Closes #20

Changes

• frailbox/src/logger.c — errno-aware open failures, write/flush fallback, helper accessors
• frailbox/include/logger.h — log_uses_stderr_fallback(), log_last_io_error()
• frailbox/tests/test_logger_errors.c — fallback + default stderr harness
• frailbox/docs/logger-errors.md — documents behavior
• frailbox/Makefile — make test-logger-errors target

Testing

python build.py --module compliance
cd frailbox && make test-logger-errors

Diagnostic decrypt password: 5a1b1d70a64e6b7c3943

Checklist

• Relevant modules build locally (compliance smoke + logger test target)
• Tests pass locally
• Diagnostic metadata committed
• Documentation updated
• Changes scoped to logger error handling

Solana wallet for bounty payout: Do4v7foHJvRJLpRRoGaVPWX6DDEjX3yTK7J91gpwUQpE