feat(client): report relay RTT diagnostics

Expose the active QUIC connection RTT in shared client diagnostics and in the
Windows client status line. This gives operators a live relay-path latency
signal without pretending to measure end-to-end gateway or LAN latency.

The new diagnostics field defaults to unknown when older JSON snapshots omit it,
so consumers can read pre-change snapshots without special migration code. The
user-facing diagnostics now print Relay RTT only when the client has an active
QUIC measurement.

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

Refs: PLAN.md logging and diagnostics section

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

@@ -265,8 +265,9 @@ impl ClientSession {
     }
 
     #[must_use]
-    pub const fn quic_diagnostics(&self) -> QuicDiagnostics {
+    pub fn quic_diagnostics(&self) -> QuicDiagnostics {
         QuicDiagnostics::new(true, Some(self.quic_max_datagram_size))
+            .with_relay_rtt_ms(Some(duration_millis_saturating(self.connection.rtt())))
     }
 
     #[must_use]
@@ -568,6 +569,10 @@ fn client_bind_addr(relay_addr: SocketAddr) -> SocketAddr {
     }
 }
 
+fn duration_millis_saturating(duration: Duration) -> u64 {
+    u64::try_from(duration.as_millis()).unwrap_or(u64::MAX)
+}
+
 async fn request_control_message(
     connection: &quinn::Connection,
     message: ControlMessage,
@@ -844,6 +849,7 @@ mod tests {
             client.quic_diagnostics().max_datagram_size(),
             Some(client.quic_max_datagram_size())
         );
+        assert!(client.quic_diagnostics().relay_rtt_ms().is_some());
         let relay_io = client.relay_io();
         assert_eq!(relay_io.welcome().peer_id(), 2);
         assert_eq!(
@@ -933,6 +939,12 @@ mod tests {
         assert_eq!(snapshot.malformed_frames(), 1);
     }
 
+    #[test]
+    fn converts_relay_rtt_to_milliseconds() {
+        assert_eq!(duration_millis_saturating(Duration::from_micros(999)), 0);
+        assert_eq!(duration_millis_saturating(Duration::from_millis(23)), 23);
+    }
+
     fn test_server_config() -> (ServerConfig, CertificateDer<'static>) {
         let certified_key =
             rcgen::generate_simple_self_signed(vec!["lanparty-relay.local".into()]).unwrap();