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-proto/src/packet.rs

@@ -613,6 +613,21 @@ pub enum SignalMessage {
         found: bool,
         relay_chain: Vec<String>,
     },
+
+    /// Request to set up a forwarding session for a specific fingerprint.
+    /// Sent over a relay link (`_relay` SNI) to ask the peer relay to
+    /// create a room and forward media for the given session.
+    SessionForward {
+        session_id: String,
+        target_fingerprint: String,
+        source_relay: String,
+    },
+    /// Confirm that the forwarding session has been set up on the peer relay.
+    /// The `room_name` tells the source relay which room to address media to.
+    SessionForwardAck {
+        session_id: String,
+        room_name: String,
+    },
 }
 
 /// Reasons for ending a call.