RelayLink: QUIC connection to peer relay (SNI "_relay") for forwarding
specific sessions. Methods: connect, forward, add/remove_session, is_idle.
RelayLinkManager: manages connections to multiple peers.
- get_or_connect: lazy connection establishment
- forward_to: send media packet to specific peer
- register/unregister_session: track which sessions use which links
- Auto-closes idle links on session unregister
Protocol: added SignalMessage::SessionForward { session_id,
target_fingerprint, source_relay } and SessionForwardAck { session_id,
room_name } for relay-link session setup signaling.
Building block for P3-T7 (call setup over mesh) which wires
route resolution + relay links + handshake into a complete flow.
62 relay tests + 42 proto tests passing (7 new relay_link tests).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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