fix: ping as engine instance method — same lifecycle as call

Ping was a static JNI method that loaded the .so before nativeInit,
crashing jemalloc. Now ping is an instance method on WzpEngine:

- Engine is created once (nativeInit), reused for both ping and call
- pingRelay() uses same tokio runtime pattern as startCall()
- Auto-pings all servers on app launch (after engine init)
- No process restart needed
- TOFU fingerprints saved on first successful ping

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

android/app/src/main/java/com/wzp/net/RelayPinger.kt

@@ -1,36 +1,12 @@
 package com.wzp.net
 
-/**
- * Relay ping via native QUIC — requires loading the native .so.
- * After ping completes, the process must be restarted (System.exit)
- * because jemalloc initialization during .so load corrupts state
- * on Android 16 MTE devices.
- *
- * Flow: ping all servers → save results → exit → app restarts → load results
- */
-object RelayPinger {
+// Relay pinging is now done via WzpEngine.pingRelay() (instance method).
+// This file kept for the data class only.
 
+object RelayPinger {
     data class PingResult(
         val rttMs: Int,
         val reachable: Boolean,
         val serverFingerprint: String = "",
     )
-
-    /**
-     * Ping a relay via the native QUIC stack.
-     * WARNING: After calling this, the process must be restarted.
-     */
-    fun ping(address: String): PingResult {
-        return try {
-            val json = com.wzp.engine.WzpEngine.pingRelay(address) ?: return PingResult(0, false)
-            val obj = org.json.JSONObject(json)
-            PingResult(
-                rttMs = obj.getInt("rtt_ms"),
-                reachable = true,
-                serverFingerprint = obj.optString("server_fingerprint", ""),
-            )
-        } catch (e: Exception) {
-            PingResult(0, false)
-        }
-    }
 }