Handle frailbox logger file failures

[xxCodexIAxx]

Summary

• Improves frailbox legacy logger handling for log file open, buffering, write, flush, and close failures.
• Falls back to stderr with a clear diagnostic instead of dropping logs or leaving a broken file stream active.
• Adds a fallback counter for diagnostics and test verification.
• Adds a small C harness plus make test-logger-errors.
• Documents the fallback behavior in operations docs.

Changes

• frailbox/src/logger.c
  • Adds a shared fallback helper for file operation failures.
  • Detects fputs / fflush failures and switches future writes to stderr.
  • Reports close/flush shutdown failures without aborting.
  • Adds log_get_fallback_count().
• frailbox/include/logger.h
  • Declares the fallback counter API.
• frailbox/tests/test_logger_errors.c
  • Verifies normal file logging, open failure fallback, and write failure fallback.
• frailbox/Makefile
  • Adds make test-logger-errors.
• docs/OPERATIONS.md
  • Documents legacy logger fallback behavior and the focused harness.

Testing

• Intended focused test: cd frailbox && make test-logger-errors.
• Intended full validation: python3 build.py.
• I could not compile or run C tests locally because this environment does not have gcc available, and full repository execution is blocked by local safety policy.

Bounty

Fixes #20.

Payout wallet: 0x776CbBb63D81c08B4090DB503eAE7FDb8fBd7c6f