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-obs/src/lib.rs

@@ -258,6 +258,8 @@ impl RelayDiagnostics {
 pub struct QuicDiagnostics {
     datagram_supported: bool,
     max_datagram_size: Option<u16>,
+    #[serde(default)]
+    relay_rtt_ms: Option<u64>,
 }
 
 impl QuicDiagnostics {
@@ -266,9 +268,16 @@ impl QuicDiagnostics {
         Self {
             datagram_supported,
             max_datagram_size,
+            relay_rtt_ms: None,
         }
     }
 
+    #[must_use]
+    pub const fn with_relay_rtt_ms(mut self, relay_rtt_ms: Option<u64>) -> Self {
+        self.relay_rtt_ms = relay_rtt_ms;
+        self
+    }
+
     #[must_use]
     pub const fn datagram_supported(&self) -> bool {
         self.datagram_supported
@@ -278,6 +287,11 @@ impl QuicDiagnostics {
     pub const fn max_datagram_size(&self) -> Option<u16> {
         self.max_datagram_size
     }
+
+    #[must_use]
+    pub const fn relay_rtt_ms(&self) -> Option<u64> {
+        self.relay_rtt_ms
+    }
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, serde::Deserialize, serde::Serialize)]
@@ -408,6 +422,13 @@ impl ClientDiagnostics {
             diagnostics.push(UserDiagnostic::new(UserDiagnosticLevel::Info, message));
         }
 
+        if let Some(rtt_ms) = self.quic.relay_rtt_ms() {
+            diagnostics.push(UserDiagnostic::new(
+                UserDiagnosticLevel::Info,
+                format!("Relay RTT: {rtt_ms} ms"),
+            ));
+        }
+
         if self.stats.broadcast_frames_tx() > 0 || self.stats.broadcast_frames_rx() > 0 {
             diagnostics.push(UserDiagnostic::new(
                 UserDiagnosticLevel::Info,
@@ -537,6 +558,7 @@ mod tests {
         assert!(diagnostics.relay().gateway_connected());
         assert!(diagnostics.quic().datagram_supported());
         assert_eq!(diagnostics.quic().max_datagram_size(), Some(1400));
+        assert_eq!(diagnostics.quic().relay_rtt_ms(), None);
         assert!(diagnostics.tap().adapter_found());
         assert_eq!(diagnostics.tap().mac(), Some(mac));
         assert_eq!(diagnostics.tap().mtu(), Some(1200));
@@ -564,11 +586,24 @@ mod tests {
         assert_eq!(stats.broadcast_frames_rx(), 0);
     }
 
+    #[test]
+    fn defaults_missing_relay_rtt_to_unknown() {
+        let diagnostics: QuicDiagnostics = serde_json::from_str(
+            r#"{
+                "datagram_supported": true,
+                "max_datagram_size": 1400
+            }"#,
+        )
+        .unwrap();
+
+        assert_eq!(diagnostics.relay_rtt_ms(), None);
+    }
+
     #[test]
     fn derives_user_diagnostics_from_client_snapshot() {
         let diagnostics = ClientDiagnostics::new(
             RelayDiagnostics::new(true, true, true),
-            QuicDiagnostics::new(true, Some(1400)),
+            QuicDiagnostics::new(true, Some(1400)).with_relay_rtt_ms(Some(23)),
             TapDiagnostics::new(
                 true,
                 Some(MacAddr::new([0x02, 1, 2, 3, 4, 5])),
@@ -590,6 +625,7 @@ mod tests {
                 "Connected to relay",
                 "Connected to LAN gateway",
                 "DHCP received: 10.73.42.51",
+                "Relay RTT: 23 ms",
                 "Broadcast traffic flowing",
             ]
         );