Improve frailbox logger error handling

[albertoblancas123]

Closes #20.

Summary:

• Falls back to stderr with actionable diagnostics when LOG_FILE cannot be opened, written, or flushed.
• Preserves the triggering log line after a file write/flush failure.
• Adds make test-logger-error-handling with a focused C harness for open-failure and /dev/full write-failure paths.
• Documents the fallback behavior in docs/OPERATIONS.md.

Validation:

• Static brace/balance review passed locally.
• Local compile not run because this Windows workspace does not provide gcc; the PR adds the focused Linux test target for maintainer/CI execution.
• Added repository-required diagnostic files under diagnostic/.