feat: P3-T6 per-session forwarding — relay links for hop-by-hop media

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>

crates/wzp-client/src/featherchat.rs

@@ -107,6 +107,8 @@ pub fn signal_to_call_type(signal: &SignalMessage) -> CallSignalType {
         SignalMessage::PresenceUpdate { .. } => CallSignalType::Offer, // reuse
         SignalMessage::RouteQuery { .. } => CallSignalType::Offer, // reuse
         SignalMessage::RouteResponse { .. } => CallSignalType::Offer, // reuse
+        SignalMessage::SessionForward { .. } => CallSignalType::Offer, // reuse
+        SignalMessage::SessionForwardAck { .. } => CallSignalType::Offer, // reuse
     }
 }