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``.