feat(relay): log egress datagram budget skips

PLAN.md keeps MVP forwarding to one Ethernet frame per QUIC datagram with no
fragmentation. The relay already skipped targets whose negotiated datagram
budget could not carry an encoded forwarded frame, but that skip was silent.

Retain the no-fragmentation behavior and log the skipped egress target with the
room, ingress peer, target peer, encoded length, and target datagram budget.
Store the peer id in relay sessions so the diagnostic can identify the skipped
target directly.

Document the egress budget-skip diagnostic in the relay README section.

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

Refs: PLAN.md

crates/lanparty-relay/src/server.rs

@@ -56,6 +56,7 @@ impl PeerKey {
 #[derive(Debug, Clone)]
 struct PeerSession {
     connection: quinn::Connection,
+    peer_id: u32,
     max_datagram_size: usize,
     latest_stats: Option<TunnelStats>,
 }
@@ -299,6 +300,7 @@ async fn register_peer(
         PeerKey::from_accepted(accepted),
         PeerSession {
             connection,
+            peer_id: accepted.peer.peer_id(),
             max_datagram_size: accepted.max_datagram_size,
             latest_stats: None,
         },
@@ -429,6 +431,16 @@ async fn forward_peer_datagram(
 
     for target in target_sessions {
         if outgoing.len() > target.max_datagram_size {
+            eprintln!(
+                "{}",
+                egress_budget_skip_log_line(
+                    &accepted.room,
+                    accepted.peer.peer_id(),
+                    target.peer_id,
+                    outgoing.len(),
+                    target.max_datagram_size,
+                )
+            );
             continue;
         }
 
@@ -590,6 +602,19 @@ fn malformed_datagram_log_line(
     )
 }
 
+fn egress_budget_skip_log_line(
+    room: &RoomCode,
+    ingress_peer_id: u32,
+    target_peer_id: u32,
+    datagram_len: usize,
+    max_datagram_size: usize,
+) -> String {
+    format!(
+        "relay egress skipped room={} peer_id={} target_peer_id={} len={} max_datagram_size={} reason=datagram_budget",
+        room, ingress_peer_id, target_peer_id, datagram_len, max_datagram_size
+    )
+}
+
 async fn collect_target_sessions(
     sessions: &Arc<Mutex<HashMap<PeerKey, PeerSession>>>,
     room: &RoomCode,
@@ -1030,6 +1055,21 @@ mod tests {
         assert!(line.contains("disconnecting=true"));
     }
 
+    #[test]
+    fn formats_egress_budget_skip_log_line() {
+        let room = RoomCode::new("TESTROOM").unwrap();
+
+        let line = egress_budget_skip_log_line(&room, 2, 7, 1401, 1400);
+
+        assert!(line.contains("relay egress skipped"));
+        assert!(line.contains("room=TESTROOM"));
+        assert!(line.contains("peer_id=2"));
+        assert!(line.contains("target_peer_id=7"));
+        assert!(line.contains("len=1401"));
+        assert!(line.contains("max_datagram_size=1400"));
+        assert!(line.contains("reason=datagram_budget"));
+    }
+
     #[tokio::test]
     async fn classifies_bad_peer_datagrams_as_malformed() {
         let rooms = Arc::new(Mutex::new(RoomRegistry::default()));