Prove the receiver end to end before any real call depends on it.
The sample rides the exact production mechanics - same envelope keys, same compact
key-ordered serialization, same Vocapable-Signature scheme signed with the same
secrets (both, during a rotation overlap), same 10-second success rule - so a
receiver that verifies this ping verifies real traffic. data.test: true is the
marker; nothing durable is written and no outbox row is minted.