feat(relay): notify peers when room members leave

The relay now sends a reliable PeerLeft control event to remaining room peers
after removing the departed peer from session and room state. Normal connection
closure is reported as Normal, while malformed-datagram disconnects are reported
as ProtocolError.

This completes the first relay-side lifecycle event pair. Delivery remains
best-effort and client-side event consumption is intentionally left for a
separate slice.

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

@@ -3,7 +3,7 @@ use std::{fs, net::SocketAddr, path::Path, sync::Arc};
 use anyhow::{Context, Result, anyhow};
 use bytes::Bytes;
 use lanparty_ctrl::{
-    CONTROL_LENGTH_PREFIX_LEN, ControlCodecError, ControlMessage, EndpointHello,
+    CONTROL_LENGTH_PREFIX_LEN, ControlCodecError, ControlMessage, DisconnectReason, EndpointHello,
     MAX_CONTROL_MESSAGE_LEN, PeerInfo, RELAY_ALPN, Reject, RejectReason, Role, RoomCode,
     ServerWelcome, decode_control_frame, encode_control_message,
 };
@@ -65,6 +65,28 @@ enum PeerDatagramOutcome {
     Malformed,
 }
 
+#[derive(Debug, Clone, PartialEq, Eq)]
+struct PeerClose {
+    reason: DisconnectReason,
+    message: String,
+}
+
+impl PeerClose {
+    fn normal(message: impl Into<String>) -> Self {
+        Self {
+            reason: DisconnectReason::Normal,
+            message: message.into(),
+        }
+    }
+
+    fn protocol_error(message: impl Into<String>) -> Self {
+        Self {
+            reason: DisconnectReason::ProtocolError,
+            message: message.into(),
+        }
+    }
+}
+
 #[derive(Debug, Default)]
 struct MalformedDatagramTracker {
     count: usize,
@@ -203,13 +225,20 @@ async fn handle_incoming_connection(
         accepted.welcome.effective_tap_mtu()
     );
 
-    let close_reason = run_peer_datagrams(&rooms, &sessions, &accepted, &connection).await;
-    leave_peer(&rooms, &sessions, &accepted.room, accepted.peer.peer_id()).await?;
+    let close = run_peer_datagrams(&rooms, &sessions, &accepted, &connection).await;
+    let leave = leave_peer(&rooms, &sessions, &accepted.room, accepted.peer.peer_id()).await?;
+    notify_peer_left(
+        &sessions,
+        &accepted.room,
+        leave.peer().peer_id(),
+        close.reason.clone(),
+    )
+    .await;
     println!(
         "peer {} left room {}: {}",
         accepted.peer.peer_id(),
         accepted.room,
-        close_reason
+        close.message
     );
 
     Ok(Some(accepted))
@@ -268,7 +297,7 @@ async fn run_peer_datagrams(
     sessions: &Arc<Mutex<HashMap<PeerKey, PeerSession>>>,
     accepted: &AcceptedPeer,
     connection: &quinn::Connection,
-) -> String {
+) -> PeerClose {
     let mut malformed_tracker = MalformedDatagramTracker::default();
 
     loop {
@@ -279,7 +308,7 @@ async fn run_peer_datagrams(
                     Ok(PeerDatagramOutcome::Malformed) => {
                         if let Some(reason) = malformed_tracker.record_malformed() {
                             connection.close(0_u32.into(), reason.as_bytes());
-                            return reason;
+                            return PeerClose::protocol_error(reason);
                         }
                     }
                     Err(error) => {
@@ -291,7 +320,7 @@ async fn run_peer_datagrams(
                     }
                 }
             }
-            Err(error) => return error.to_string(),
+            Err(error) => return PeerClose::normal(error.to_string()),
         }
     }
 }
@@ -450,6 +479,22 @@ async fn notify_peer_joined(
     }
 }
 
+async fn notify_peer_left(
+    sessions: &Arc<Mutex<HashMap<PeerKey, PeerSession>>>,
+    room: &RoomCode,
+    peer_id: u32,
+    reason: DisconnectReason,
+) {
+    let target_sessions = collect_room_sessions_except(sessions, room, peer_id).await;
+    let message = ControlMessage::PeerLeft { peer_id, reason };
+
+    for target in target_sessions {
+        if let Err(error) = send_control_event(&target.connection, &message).await {
+            eprintln!("failed to notify room {room} about peer {peer_id} leaving: {error:#}");
+        }
+    }
+}
+
 async fn collect_room_sessions_except(
     sessions: &Arc<Mutex<HashMap<PeerKey, PeerSession>>>,
     room: &RoomCode,
@@ -579,18 +624,18 @@ async fn leave_peer(
     sessions: &Arc<Mutex<HashMap<PeerKey, PeerSession>>>,
     room: &RoomCode,
     peer_id: u32,
-) -> Result<()> {
+) -> Result<crate::LeaveResult> {
     sessions
         .lock()
         .await
         .remove(&PeerKey::new(room.clone(), peer_id));
-    rooms
+    let leave = rooms
         .lock()
         .await
         .leave(room, peer_id)
         .with_context(|| format!("failed to remove peer {peer_id} from room {room}"))?;
 
-    Ok(())
+    Ok(leave)
 }
 
 fn write_development_certificate(path: &Path, certificate: &CertificateDer<'_>) -> Result<()> {
@@ -883,6 +928,13 @@ mod tests {
         assert_eq!(first_welcome.room_id(), second_welcome.room_id());
 
         first_connection.close(0_u32.into(), b"test complete");
+        let event = read_control_event(&second_connection).await;
+        let ControlMessage::PeerLeft { peer_id, reason } = event else {
+            panic!("expected peer left event");
+        };
+        assert_eq!(peer_id, first_welcome.peer_id());
+        assert_eq!(reason, DisconnectReason::Normal);
+
         second_connection.close(0_u32.into(), b"test complete");
         first_endpoint.wait_idle().await;
         second_endpoint.wait_idle().await;