Add logger rotation retention report fixture

[h1065153539-create]

Summary

• Adds a legacy logger retention-report helper that emits JSON rotation decision metadata.
• Reports retained and pruned counts plus per-file name, size, optional mtime, decision, and reason.
• Redacts secret-like metadata names/reasons and never reads log file contents.
• Adds a lightweight C fixture and Makefile target for retention decisions, plus operations documentation.

Validation

• make -C frailbox clean logger-retention-fixtures passed.
• make -C frailbox all passed, with the existing unrelated arena.c pointer-type warning.
• python3 build.py -m frailbox compiled frailbox successfully in the API-sourced worktree; the encrypted diagnostic log records the expected non-git diagnostic commit limitation for this local checkout.

Diagnostic bundle

• diagnostic/build-44ce4ece.logd
• diagnostic/build-44ce4ece.json

Closes #3.

[Soengkit]

Closing this PR because the linked fork issue is only a closed payout tracker for an already submitted upstream PR. This fork is not an active bounty intake or payment authority, and it is not accepting external submissions or payment details here.