3 new test files, 24 cases (8 per validator):
- baseline (no annotation fields) still valid
- revision: 0 / revision: 5 accepted
- source_annotations list-of-dict accepted
- annotation_digest string accepted
- revision_reason accepted
- all 4 fields together accepted
- unrecognized future field tolerated (forward-compat policy)
Pin against future strict-key refactors. No production code change — pure regression pin.
