[$25 BOUNTY] [C] Add frailbox logger rotation retention report

[leo202000]

Summary

Adds an audit-friendly retention report for frailbox logger rotation decisions, as requested in issue #3.

Changes

• frailbox/include/logger.h: New rotation retention report API:

  • log_rotation_report() — scans *.log files in a directory and produces retain/prune decisions based on max_files, max_age_secs, and max_total_bytes policies.
  • log_rotation_report_to_json() / log_rotation_report_to_text() — formatters with ISO-8601 mtime timestamps.
  • log_rotation_report_free() — cleanup.
  • Only file metadata is accessed (name, size, mtime); raw log content is never read or exposed.

• frailbox/src/logger.c: Implementation of the retention report. Policies applied in order: age → count → total size (oldest pruned first). Entries sorted newest-first.

• frailbox/tests/test_rotation.c (new): 30 tests covering:

  • max_files policy (retain 2 newest, prune 3 oldest)
  • max_age policy (prune files older than threshold)
  • Combined age + count policy
  • No-limits (all retained)
  • JSON/text output format validation
  • Secret-leak prevention (verifies JSON/text never contains raw log content)
  • NULL and error handling
  • JSON field completeness (filename, size, mtime, decision, reason)

• frailbox/Makefile: Added test_rotation target.

• docs/OPERATIONS.md: Documented retention report usage, policies, and JSON format.

Testing

• make test_rotation — 30 tests, all pass (WSL Ubuntu, gcc 13.3.0)
• python3 build.py — diagnostic build log generated and included below
  • Build results: frontend PASS, frailbox PASS, engine PASS (3/10 modules; 7 FAIL due to missing toolchains: cargo/go/javac/ruby/luac/ghc)

Checklist

• Relevant modules affected by these changes build locally
• Tests pass locally
• Diagnostic build log is committed in this PR (diagnostic/build-c68dc00f.logd)
• Changes are scoped to the PR purpose and avoid unrelated cleanup
• Security, privacy, and error-handling implications have been considered

Closes #3