perhapsDecode fell back to the not-yet-verified key held during a key
verification handshake for any incoming PKI unicast. That key is supplied
by whoever opened the handshake and proves only that they hold it, not that
they are the node they claim to be, so until the session ended they could
send DMs on any port that decrypted and were marked pki_encrypted.
perhapsEncode already restricts the pending key to KEY_VERIFICATION_APP.
The receive path now applies the same rule.