test(relay): cover real client and gateway sessions

Add a relay-server test that connects the production client-core session and the
production gateway session to the same in-process relay. The test verifies room
join catch-up, negotiated peer identities, Ethernet datagrams in both
directions, stats reporting, and room/session cleanup after graceful shutdown.

The existing relay tests covered forwarding with raw Quinn peers. This adds a
higher-level proof that the three MVP components agree on the same control and
datagram contracts before the final Windows TAP and physical LAN test.

Test Plan:
- cargo fmt --check
- cargo test -p lanparty-relay bridges_real_client_and_gateway_sessions -- --nocapture
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check

Refs: PLAN.md Phase 1 relay/client/gateway proof

crates/lanparty-relay/src/server.rs

@@ -905,7 +905,9 @@ mod tests {
     };
 
     use bytes::Bytes;
+    use lanparty_client_core::{ClientSessionConfig, connect_client};
     use lanparty_ctrl::{RoomCode, decode_control_frame, encode_control_message};
+    use lanparty_gateway::{GatewayConfig, connect_gateway};
     use lanparty_proto::{FrameType, MacAddr, decode_datagram, encode_datagram};
     use quinn::{ClientConfig, crypto::rustls::QuicClientConfig};
 
@@ -1223,6 +1225,139 @@ mod tests {
         assert!(sessions.lock().await.is_empty());
     }
 
+    #[tokio::test]
+    async fn bridges_real_client_and_gateway_sessions() {
+        let (server, certificate) = bind_test_server(DEFAULT_MAX_CLIENTS_PER_ROOM);
+        let rooms = Arc::clone(&server.rooms);
+        let sessions = Arc::clone(&server.sessions);
+        let server_addr = server.local_addr().unwrap();
+        let server_task = tokio::spawn(async move {
+            let handles = server.accept_many_for_test(2).await.unwrap();
+            let mut accepted = Vec::with_capacity(handles.len());
+
+            for handle in handles {
+                accepted.push(handle.await.unwrap().unwrap().unwrap());
+            }
+
+            server.shutdown("test complete").await;
+            accepted
+        });
+        let cert_der = certificate.as_ref().to_vec();
+        let room = RoomCode::new("TESTROOM").unwrap();
+        let client_mac = client_mac(1);
+        let gateway = connect_gateway(
+            GatewayConfig::new(
+                server_addr,
+                "lanparty-relay.local",
+                cert_der.clone(),
+                room.clone(),
+                "eth0",
+                1400,
+            )
+            .unwrap(),
+        )
+        .await
+        .unwrap();
+        let client = connect_client(
+            ClientSessionConfig::new(
+                server_addr,
+                "lanparty-relay.local",
+                cert_der,
+                room.clone(),
+                client_mac,
+                1400,
+            )
+            .unwrap(),
+        )
+        .await
+        .unwrap();
+
+        assert_eq!(gateway.welcome().room_id(), client.welcome().room_id());
+        assert_eq!(gateway.welcome().peer_id(), 1);
+        assert_eq!(client.welcome().peer_id(), 2);
+        assert!(client.welcome().gateway_connected());
+        assert_eq!(
+            gateway.quic_max_datagram_size(),
+            client.quic_max_datagram_size()
+        );
+
+        let ControlMessage::PeerJoined(peer) =
+            tokio::time::timeout(Duration::from_secs(5), gateway.recv_control_event())
+                .await
+                .unwrap()
+                .unwrap()
+        else {
+            panic!("expected gateway to observe client join");
+        };
+        assert_eq!(peer.peer_id(), client.welcome().peer_id());
+        assert_eq!(peer.role(), Role::Client);
+        assert_eq!(peer.mac(), Some(client_mac));
+
+        let ControlMessage::PeerJoined(peer) =
+            tokio::time::timeout(Duration::from_secs(5), client.recv_control_event())
+                .await
+                .unwrap()
+                .unwrap()
+        else {
+            panic!("expected client to receive gateway catch-up event");
+        };
+        assert_eq!(peer.peer_id(), gateway.welcome().peer_id());
+        assert_eq!(peer.role(), Role::Gateway);
+
+        let client_to_lan = ethernet_frame(gateway_mac(), client_mac);
+        assert_eq!(
+            client
+                .relay_io()
+                .send_ethernet_with_outcome(&client_to_lan)
+                .unwrap(),
+            lanparty_client_core::ClientSendOutcome::Sent
+        );
+        let received = tokio::time::timeout(Duration::from_secs(5), gateway.recv_ethernet())
+            .await
+            .unwrap()
+            .unwrap();
+        assert_eq!(received.source_peer_id(), client.welcome().peer_id());
+        assert_eq!(received.payload(), client_to_lan.as_slice());
+
+        let lan_to_client = ethernet_frame(client_mac, gateway_mac());
+        gateway.send_ethernet(&lan_to_client).unwrap();
+        let received =
+            tokio::time::timeout(Duration::from_secs(5), client.relay_io().recv_ethernet())
+                .await
+                .unwrap()
+                .unwrap();
+        assert_eq!(received.source_peer_id(), gateway.welcome().peer_id());
+        assert_eq!(received.payload(), lan_to_client.as_slice());
+
+        client.send_stats_snapshot().await.unwrap();
+        gateway.send_stats_snapshot().await.unwrap();
+        wait_for_peer_stats(
+            &sessions,
+            &room,
+            client.welcome().peer_id(),
+            &client.stats_snapshot(),
+        )
+        .await;
+        wait_for_peer_stats(
+            &sessions,
+            &room,
+            gateway.welcome().peer_id(),
+            &gateway.stats_snapshot(),
+        )
+        .await;
+
+        client.shutdown("test client done").await;
+        gateway.shutdown("test gateway done").await;
+
+        let accepted = tokio::time::timeout(Duration::from_secs(5), server_task)
+            .await
+            .unwrap()
+            .unwrap();
+        assert_eq!(accepted.len(), 2);
+        assert_eq!(rooms.lock().await.room_count(), 0);
+        assert!(sessions.lock().await.is_empty());
+    }
+
     #[tokio::test]
     async fn forwards_graceful_disconnect_reason_to_remaining_peers() {
         let (server, certificate) = bind_test_server(DEFAULT_MAX_CLIENTS_PER_ROOM);
@@ -1422,6 +1557,10 @@ mod tests {
         MacAddr::new([0x02, 0, 0, 0, 0, last])
     }
 
+    fn gateway_mac() -> MacAddr {
+        MacAddr::new([0x0a, 0, 0, 0, 0, 1])
+    }
+
     fn ethernet_frame(destination: MacAddr, source: MacAddr) -> Vec<u8> {
         let mut frame = Vec::new();
         frame.extend_from_slice(&destination.octets());