feat: ping-and-exit for server RTT, remove broken UDP ping

- Ping button: pings all servers via native QUIC, saves RTT + fingerprint
  to SharedPreferences, then exits process (System.exit)
- On restart: loads saved ping results (no native .so loading needed)
- Avoids jemalloc crash: native lib only loaded once per process lifetime
- Removed broken UDP probe (QUIC servers don't respond to it)
- SettingsRepository: savePingRtt/loadPingRtt for cached results
- PingResult: added reachable field

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

android/app/src/main/java/com/wzp/data/SettingsRepository.kt

@@ -185,4 +185,14 @@ class SettingsRepository(context: Context) {
     fun loadServerFingerprint(address: String): String? {
         return prefs.getString("$TOFU_PREFIX$address", null)
     }
+
+    // --- Ping RTT cache ---
+
+    fun savePingRtt(address: String, rttMs: Int) {
+        prefs.edit().putInt("ping_rtt_$address", rttMs).apply()
+    }
+
+    fun loadPingRtt(address: String): Int {
+        return prefs.getInt("ping_rtt_$address", -1)
+    }
 }