## Summary
- Adds `SnapshotExpirationRefsTest` with 3 integration tests validating
how Iceberg snapshot expiration interacts with tags and branches
- Demonstrates that refs protect snapshots from expiration (compliance
gap)
- Validates that `history.expire.max-ref-age-ms` table property causes
`expire_snapshots` to drop expired refs and their protected snapshots

## Test plan
- [x] Tests pass locally with Java 17
- [ ] CI passes