feat: relay ping with RTT, server TOFU, lock icons (Phase 2 backport)

Rust JNI:
- nativePingRelay: QUIC connect with 3s timeout, returns RTT + server
  certificate fingerprint as JSON. Static method, no engine needed.

Kotlin:
- WzpEngine.pingRelay() static wrapper
- SettingsRepository: TOFU fingerprint persistence (tofu_{address} keys)
- CallViewModel: pingAllServers() coroutine, lockStatus() helper,
  PingResult/LockStatus data types
- InCallScreen: server chips show lock icon + RTT color (green/yellow),
  "Ping All" button

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-android/src/jni_bridge.rs

@@ -317,3 +317,79 @@ pub unsafe extern "system" fn Java_com_wzp_engine_WzpEngine_nativeDestroy(
         drop(h);
     }));
 }
+
+/// Ping a relay server — returns JSON `{"rtt_ms":N,"server_fingerprint":"hex"}` or null on failure.
+/// Does NOT require an engine handle — creates a temporary QUIC connection.
+#[unsafe(no_mangle)]
+pub unsafe extern "system" fn Java_com_wzp_engine_WzpEngine_nativePingRelay<'a>(
+    mut env: JNIEnv<'a>,
+    _class: JClass,
+    relay_j: JString,
+) -> jstring {
+    let result = panic::catch_unwind(panic::AssertUnwindSafe(|| {
+        let relay: String = env.get_string(&relay_j).map(|s| s.into()).unwrap_or_default();
+        let addr: std::net::SocketAddr = match relay.parse() {
+            Ok(a) => a,
+            Err(_) => return None,
+        };
+
+        let _ = rustls::crypto::ring::default_provider().install_default();
+
+        let rt = match tokio::runtime::Builder::new_current_thread()
+            .enable_all()
+            .build()
+        {
+            Ok(rt) => rt,
+            Err(_) => return None,
+        };
+
+        rt.block_on(async {
+            let bind: std::net::SocketAddr = "0.0.0.0:0".parse().unwrap();
+            let endpoint = match wzp_transport::create_endpoint(bind, None) {
+                Ok(e) => e,
+                Err(_) => return None,
+            };
+            let client_cfg = wzp_transport::client_config();
+            let start = std::time::Instant::now();
+
+            match tokio::time::timeout(
+                std::time::Duration::from_secs(3),
+                wzp_transport::connect(&endpoint, addr, "ping", client_cfg),
+            )
+            .await
+            {
+                Ok(Ok(conn)) => {
+                    let rtt_ms = start.elapsed().as_millis() as u64;
+                    let server_fp = conn
+                        .peer_identity()
+                        .and_then(|id| {
+                            id.downcast::<Vec<rustls::pki_types::CertificateDer>>().ok()
+                        })
+                        .and_then(|certs| {
+                            certs.first().map(|c| {
+                                use std::hash::{Hash, Hasher};
+                                let mut h = std::collections::hash_map::DefaultHasher::new();
+                                c.as_ref().hash(&mut h);
+                                format!("{:016x}", h.finish())
+                            })
+                        })
+                        .unwrap_or_default();
+                    conn.close(0u32.into(), b"ping");
+                    Some(format!(
+                        r#"{{"rtt_ms":{},"server_fingerprint":"{}"}}"#,
+                        rtt_ms, server_fp
+                    ))
+                }
+                _ => None,
+            }
+        })
+    }));
+
+    let json = match result {
+        Ok(Some(s)) => s,
+        _ => return JObject::null().into_raw(),
+    };
+    env.new_string(&json)
+        .map(|s| s.into_raw())
+        .unwrap_or(JObject::null().into_raw())
+}