feat(client): receive relay control events

ClientSession can now accept one-frame relay control events sent on reliable
unidirectional QUIC streams. This gives the Windows client a core API for the
PeerJoined and PeerLeft lifecycle messages that the relay now emits.

The implementation stays in client-core because it shares the relay connection
and control codec with the handshake. Client UI/status handling remains a
separate slice so this commit only establishes the tested transport boundary.

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

Refs: PLAN.md

crates/lanparty-client-core/src/lib.rs

@@ -274,6 +274,10 @@ impl ClientSession {
         self.relay_io().recv_ethernet().await
     }
 
+    pub async fn recv_control_event(&self) -> Result<ControlMessage> {
+        recv_control_event(&self.connection).await
+    }
+
     #[must_use]
     pub fn stats_snapshot(&self) -> TunnelStats {
         self.stats.snapshot()
@@ -496,12 +500,25 @@ async fn request_control_message(
     Ok(decode_control_frame(&response)?)
 }
 
+async fn recv_control_event(connection: &quinn::Connection) -> Result<ControlMessage> {
+    let mut recv = connection
+        .accept_uni()
+        .await
+        .context("failed to accept relay control event stream")?;
+    let frame = recv
+        .read_to_end(MAX_CONTROL_FRAME_LEN)
+        .await
+        .context("failed to read relay control event")?;
+
+    decode_control_frame(&frame).context("failed to decode relay control event")
+}
+
 #[cfg(test)]
 mod tests {
     use std::time::{Duration, SystemTime, UNIX_EPOCH};
 
     use bytes::Bytes;
-    use lanparty_ctrl::Role;
+    use lanparty_ctrl::{PeerInfo, Role};
     use quinn::{ServerConfig, TransportConfig, crypto::rustls::QuicServerConfig};
     use rustls::pki_types::{PrivateKeyDer, PrivatePkcs8KeyDer};
 
@@ -652,6 +669,14 @@ mod tests {
                     .unwrap();
             connection.send_datagram(Bytes::from(response)).unwrap();
 
+            let event = encode_control_message(&ControlMessage::PeerJoined(
+                PeerInfo::new(1, Role::Gateway, None).unwrap(),
+            ))
+            .unwrap();
+            let mut event_send = connection.open_uni().await.unwrap();
+            event_send.write_all(&event).await.unwrap();
+            event_send.finish().unwrap();
+
             connection.closed().await;
             endpoint.close(0_u32.into(), b"test complete");
             endpoint.wait_idle().await;
@@ -694,6 +719,16 @@ mod tests {
         assert_eq!(received.source_peer_id(), 1);
         assert_eq!(received.payload(), ethernet_frame(b"from relay").as_slice());
 
+        let event = tokio::time::timeout(Duration::from_secs(5), client.recv_control_event())
+            .await
+            .unwrap()
+            .unwrap();
+        let ControlMessage::PeerJoined(peer) = event else {
+            panic!("expected peer joined event");
+        };
+        assert_eq!(peer.peer_id(), 1);
+        assert_eq!(peer.role(), Role::Gateway);
+
         assert!(relay_io.send_ethernet(&[0; 4]).is_err());
         let stats = relay_io.stats_snapshot();
         assert_eq!(stats.ethernet_frames_tx(), 1);