feat(client): identify lifecycle leave events

The relay now sends enough PeerJoined events for the client to learn the peers
already in the room, but the client was still formatting PeerLeft with only the
bare peer id. That made a gateway disconnect look the same as any other peer
leaving.

Keep a small in-memory peer map in the control-event logger. PeerJoined records
or refreshes the peer identity, and PeerLeft removes it before formatting the
message. Unknown leaves still use the generic fallback, so out-of-order or
missed events remain understandable.

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

Refs: PLAN.md

crates/lanparty-client-win/src/main.rs

@@ -1,6 +1,6 @@
 #[cfg(windows)]
 use std::net::IpAddr;
-use std::{fs, net::SocketAddr, path::PathBuf};
+use std::{collections::BTreeMap, fs, net::SocketAddr, path::PathBuf};
 #[cfg(windows)]
 use std::{
     sync::{Arc, mpsc},
@@ -22,7 +22,7 @@ use lanparty_client_route::{
 };
 #[cfg(windows)]
 use lanparty_client_tap::TapAdapter;
-use lanparty_ctrl::{ControlMessage, Role, RoomCode};
+use lanparty_ctrl::{ControlMessage, PeerInfo, Role, RoomCode};
 use lanparty_obs::{ClientDiagnostics, RelayDiagnostics, TapDiagnostics};
 use lanparty_proto::MacAddr;
 
@@ -416,24 +416,73 @@ fn optional_label<T: std::fmt::Display>(value: Option<T>) -> String {
 }
 
 async fn run_control_event_log(session: &ClientSession) -> Result<()> {
+    let mut formatter = ControlEventFormatter::default();
+
     loop {
         let event = session
             .recv_control_event()
             .await
             .context("failed to receive relay control event")?;
-        println!("{}", format_control_event(&event));
+        println!("{}", formatter.format(&event));
     }
 }
 
+#[cfg(test)]
 fn format_control_event(event: &ControlMessage) -> String {
-    match event {
-        ControlMessage::PeerJoined(peer) if peer.role() == Role::Gateway => {
+    ControlEventFormatter::default().format(event)
+}
+
+#[derive(Debug, Default)]
+struct ControlEventFormatter {
+    peers: BTreeMap<u32, PeerInfo>,
+}
+
+impl ControlEventFormatter {
+    fn format(&mut self, event: &ControlMessage) -> String {
+        match event {
+            ControlMessage::PeerJoined(peer) => {
+                self.peers.insert(peer.peer_id(), peer.clone());
+                format_peer_joined(peer)
+            }
+            ControlMessage::PeerLeft { peer_id, reason } => {
+                let Some(peer) = self.peers.remove(peer_id) else {
+                    return format!("relay event: peer {peer_id} left ({reason:?})");
+                };
+
+                match peer.role() {
+                    Role::Gateway => {
+                        format!(
+                            "relay event: LAN gateway disconnected (peer {}, {reason:?})",
+                            peer.peer_id()
+                        )
+                    }
+                    Role::Client => {
+                        let mac = peer
+                            .mac()
+                            .map(|mac| mac.to_string())
+                            .unwrap_or_else(|| "unknown".to_string());
+                        format!(
+                            "relay event: client peer {} with MAC {} left ({reason:?})",
+                            peer.peer_id(),
+                            mac
+                        )
+                    }
+                }
+            }
+            _ => format!("relay event: {event:?}"),
+        }
+    }
+}
+
+fn format_peer_joined(peer: &PeerInfo) -> String {
+    match peer.role() {
+        Role::Gateway => {
             format!(
                 "relay event: LAN gateway connected as peer {}",
                 peer.peer_id()
             )
         }
-        ControlMessage::PeerJoined(peer) => {
+        Role::Client => {
             let mac = peer
                 .mac()
                 .map(|mac| mac.to_string())
@@ -444,10 +493,6 @@ fn format_control_event(event: &ControlMessage) -> String {
                 mac
             )
         }
-        ControlMessage::PeerLeft { peer_id, reason } => {
-            format!("relay event: peer {peer_id} left ({reason:?})")
-        }
-        _ => format!("relay event: {event:?}"),
     }
 }
 
@@ -735,22 +780,39 @@ mod tests {
         let gateway = ControlMessage::PeerJoined(PeerInfo::new(1, Role::Gateway, None).unwrap());
         let client =
             ControlMessage::PeerJoined(PeerInfo::new(2, Role::Client, Some(mac(2))).unwrap());
-        let left = ControlMessage::PeerLeft {
+        let unknown_left = ControlMessage::PeerLeft {
+            peer_id: 3,
+            reason: DisconnectReason::Normal,
+        };
+        let client_left = ControlMessage::PeerLeft {
             peer_id: 2,
             reason: DisconnectReason::Normal,
         };
+        let gateway_left = ControlMessage::PeerLeft {
+            peer_id: 1,
+            reason: DisconnectReason::Normal,
+        };
+        let mut formatter = ControlEventFormatter::default();
 
         assert_eq!(
-            format_control_event(&gateway),
+            formatter.format(&gateway),
             "relay event: LAN gateway connected as peer 1"
         );
         assert_eq!(
-            format_control_event(&client),
+            formatter.format(&client),
             "relay event: client peer 2 joined with MAC 02:00:00:00:00:02"
         );
         assert_eq!(
-            format_control_event(&left),
-            "relay event: peer 2 left (Normal)"
+            format_control_event(&unknown_left),
+            "relay event: peer 3 left (Normal)"
+        );
+        assert_eq!(
+            formatter.format(&client_left),
+            "relay event: client peer 2 with MAC 02:00:00:00:00:02 left (Normal)"
+        );
+        assert_eq!(
+            formatter.format(&gateway_left),
+            "relay event: LAN gateway disconnected (peer 1, Normal)"
         );
     }