Phase 1 of relay federation:
1. Signal messages: FederationRoomJoin/Leave/ParticipantUpdate added
to SignalMessage enum for relay-to-relay room coordination.
2. Room changes: ParticipantOrigin (Local/Federated) tracking, loop
prevention (federated media only forwards to local participants),
ParticipantSender::Federation with 8-byte room-hash prefixed
datagrams, merged participant lists (local + remote), new methods:
join_federated(), update_federated_participants(), local_senders(),
active_rooms(), local_participants().
3. FederationManager: connects to configured peers via QUIC with SNI
"_federation", reconnects with exponential backoff (5s-300s),
exchanges FederationRoomJoin signals, runs recv loops for both
signals and media datagrams, creates virtual participants in rooms.
4. Accept-side: _federation SNI handling in main.rs, unknown peer
gets helpful "add to relay.toml" log message, recognized peers
handed off to FederationManager.
TODO: TLS fingerprint verification — currently outbound connections
use client_config() which doesn't present a cert, so inbound
verification fails. Need mutual TLS or URL-based peer matching.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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