debug(media): add connection diagnostics for direct P2P drops

When direct P2P calls show 100% datagram drops, we need to know
WHY send_media() fails. This commit adds:

- Remote address + stable_id logging on A-role accept and D-role
  dial success (dual_path.rs) — tells us which candidate won
- Remote address + max_datagram_size on engine transport init —
  verifies datagrams are negotiated
- last_send_err in send heartbeat — captures the actual error
  from send_datagram() failures
- QuinnTransport::remote_address() helper

Also fixes UI badge: was looking for wrong event name
("dual_path_race_won" → "path_negotiated").

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-transport/src/quic.rs

@@ -33,6 +33,11 @@ impl QuinnTransport {
         &self.connection
     }
 
+    /// Remote address of the peer on this connection.
+    pub fn remote_address(&self) -> std::net::SocketAddr {
+        self.connection.remote_address()
+    }
+
     /// Send raw bytes as a QUIC datagram (no MediaPacket framing).
     pub fn send_raw_datagram(&self, data: &[u8]) -> Result<(), TransportError> {
         self.connection