Deliver a relayer-signed receipt to a counterparty's sealed v2 mailbox

Seals the receipt (ML-KEM-768+X25519, one-time handle) to the recipient's messaging-v2 mailbox and stores it on the same wire shape as any message, so the counterparty gets verifiable proof without contacting Robyn. Only receipts signed by this relayer are accepted. Content-hash idempotent (replay is a no-op); reachable over OHTTP.