Passthrough routing

sender --(ct, to, lookup_id)--> NATS subject
     |                              |
     |                              v
     |                         sm-leaf / NPE
     |                              |
     +---- lookup_id only ----      v
                              recipient mailbox

Header (clear): to, from_lookup_id, alg, error_token. Body: ciphertext. Broker must not log ct.