Two-step tool to verify that backfill_sidecars doesn't wipe the
bw_report block from existing sidecars. Workflow:
1. snapshot --out before.json (canonical-JSON hash per sidecar)
2. run backfill
3. diff --baseline before.json (classifies every sidecar:
PRESERVED / CHANGED / WIPED / STILL_MISSING / NEW / ADDED / REMOVED)
Exit code 1 if any WIPED or CHANGED entries found, 0 otherwise — so
it can gate a CI step or a deploy script.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
