Enforce that revisions entries are never rewritten or removed, against the base commit #333

The revisions array is documented as append-only (#317, #323). validate_records.py checks one record at a time, so it enforces date order, known field names, no-op entries, the from/to chain and agreement with the record, but it cannot see an earlier entry that was edited or deleted, or the whole array being removed. That needs the previous version of the record.

Proposal: a --ratchet-since mode like check_confidence_signal.py's, using the same three-dot diff, that fails when an entry present in the base version of a changed record is altered or missing in the head version, or when the array disappears. Scope: records changed since the base only. Same bar as #313: mutation-test it, including three-dot versus two-dot.

Not blocking #323.