A person's merge can take identity status away from the surviving entity #1426

merge_entities invalidates a duplicate name fact without copying its fact_evidence. When a person merges an entity whose name came from an identity source (ADR 0071) into one that already holds the same name from prose with an earlier recorded_at, the surviving name fact keeps only the prose evidence. The survivor is then no longer an identity entity: decision 4's exact-name attach stops applying to it, and decision 7's hold (#1425) no longer fires for it. Found while reviewing #1425. The fix is probably to move the duplicate's evidence onto the surviving fact before invalidating it, as facts moved by a merge keep theirs.