feat(relay): notify peers when rooms change

The relay now sends a reliable PeerJoined control event to peers that were
already present in the room after a new peer completes the hello/welcome
handshake. Events are sent on one-frame unidirectional QUIC streams, reusing the
existing control codec without keeping the room/session lock across I/O.

Delivery is best-effort for this first lifecycle slice: a notification failure
is logged, but the newly accepted peer remains joined. PeerLeft delivery and
client-side event consumption remain separate follow-up work.

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

Refs: PLAN.md

crates/lanparty-relay/src/server.rs

@@ -242,6 +242,10 @@ async fn accept_control_handshake(
         return Err(error);
     }
 
+    if let Some(accepted) = &accepted {
+        notify_peer_joined(sessions, accepted).await;
+    }
+
     Ok(accepted)
 }
 
@@ -427,6 +431,50 @@ async fn collect_target_sessions(
         .collect()
 }
 
+async fn notify_peer_joined(
+    sessions: &Arc<Mutex<HashMap<PeerKey, PeerSession>>>,
+    accepted: &AcceptedPeer,
+) {
+    let target_sessions =
+        collect_room_sessions_except(sessions, &accepted.room, accepted.peer.peer_id()).await;
+    let message = ControlMessage::PeerJoined(accepted.peer.clone());
+
+    for target in target_sessions {
+        if let Err(error) = send_control_event(&target.connection, &message).await {
+            eprintln!(
+                "failed to notify room {} about peer {} joining: {error:#}",
+                accepted.room,
+                accepted.peer.peer_id()
+            );
+        }
+    }
+}
+
+async fn collect_room_sessions_except(
+    sessions: &Arc<Mutex<HashMap<PeerKey, PeerSession>>>,
+    room: &RoomCode,
+    excluded_peer_id: u32,
+) -> Vec<PeerSession> {
+    let sessions = sessions.lock().await;
+
+    sessions
+        .iter()
+        .filter(|(key, _)| key.room == *room && key.peer_id != excluded_peer_id)
+        .map(|(_, session)| session.clone())
+        .collect()
+}
+
+async fn send_control_event(
+    connection: &quinn::Connection,
+    message: &ControlMessage,
+) -> Result<()> {
+    let mut send = connection
+        .open_uni()
+        .await
+        .context("failed to open relay control event stream")?;
+    send_control_message(&mut send, message).await
+}
+
 async fn build_handshake_response(
     rooms: &Arc<Mutex<RoomRegistry>>,
     connection: &quinn::Connection,
@@ -516,12 +564,12 @@ fn reject_codec_error(error: ControlCodecError) -> Reject {
 }
 
 async fn send_control_message(send: &mut SendStream, message: &ControlMessage) -> Result<()> {
-    let response = encode_control_message(message).context("failed to encode control response")?;
-    send.write_all(&response)
+    let frame = encode_control_message(message).context("failed to encode control message")?;
+    send.write_all(&frame)
         .await
-        .context("failed to write control response")?;
+        .context("failed to write control message")?;
     send.finish()
-        .map_err(|error| anyhow!("failed to finish control response stream: {error}"))?;
+        .map_err(|error| anyhow!("failed to finish control message stream: {error}"))?;
 
     Ok(())
 }
@@ -799,6 +847,13 @@ mod tests {
         let second_mac = client_mac(2);
         let first_welcome = welcome_for_client(&first_connection, first_mac).await;
         let second_welcome = welcome_for_client(&second_connection, second_mac).await;
+        let event = read_control_event(&first_connection).await;
+        let ControlMessage::PeerJoined(peer) = event else {
+            panic!("expected peer joined event");
+        };
+        assert_eq!(peer.peer_id(), second_welcome.peer_id());
+        assert_eq!(peer.role(), Role::Client);
+        assert_eq!(peer.mac(), Some(second_mac));
 
         let ethernet = ethernet_frame(second_mac, first_mac);
         let datagram = encode_datagram(
@@ -890,6 +945,16 @@ mod tests {
         Ok(decode_control_frame(&response)?)
     }
 
+    async fn read_control_event(connection: &quinn::Connection) -> ControlMessage {
+        let mut recv = tokio::time::timeout(Duration::from_secs(5), connection.accept_uni())
+            .await
+            .unwrap()
+            .unwrap();
+        let frame = recv.read_to_end(MAX_CONTROL_FRAME_LEN).await.unwrap();
+
+        decode_control_frame(&frame).unwrap()
+    }
+
     async fn welcome_for_client(connection: &quinn::Connection, mac: MacAddr) -> ServerWelcome {
         let hello = EndpointHello::client(RoomCode::new("TESTROOM").unwrap(), mac, 1400).unwrap();
         let response = request_control_message(connection, ControlMessage::Hello(hello))