Two identities in one page join the same Puddle over Conn.to_domain,
claim a nick, talk, and then build a relay-only DM out of candidates the Puddle
relayed for them. Along the way it checks the one thing the Puddle must never
get wrong: a signal whose from is not the fingerprint the daemon
authenticated is dropped, not forwarded.
Server side: dtls-proxy, swbrd-ext-puddle.
Add ?dm=0 to stop before the DM.