feat: pure Kotlin UDP ping — periodic every 5s, no JNI crash

Replace WzpEngine.pingRelay() (JNI, loads native .so, crashes jemalloc
on Android 16 MTE) with pure Kotlin DatagramSocket UDP probe.

- RelayPinger: sends QUIC Version Negotiation trigger packet, measures
  RTT from response. No native lib, no JNI, zero crash risk.
- Periodic: pings all servers every 5 seconds via coroutine
- Server fingerprint: filled lazily on first real QUIC connection
  (TOFU still works, just delayed)
- Lock status: OFFLINE when ping fails, NEW until first connection

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

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

@@ -0,0 +1,66 @@
+package com.wzp.net
+
+import android.util.Log
+import java.net.DatagramPacket
+import java.net.DatagramSocket
+import java.net.InetSocketAddress
+
+/**
+ * Pure Kotlin UDP ping — no JNI, no native lib loading.
+ * Sends a minimal packet to the relay and measures response time.
+ * QUIC servers reply with Version Negotiation to unknown packets.
+ */
+object RelayPinger {
+    private const val TAG = "RelayPinger"
+    private const val TIMEOUT_MS = 2000
+
+    // Minimal QUIC-like Initial packet (just enough to provoke a response)
+    // First byte 0xC0 = long header, version 0x00000000 = version negotiation trigger
+    private val PROBE = byteArrayOf(
+        0xC0.toByte(), // long header form
+        0x00, 0x00, 0x00, 0x00, // version 0 → triggers Version Negotiation
+        0x08, // DCID length = 8
+        0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, // fake DCID
+        0x00, // SCID length = 0
+        0x00, 0x00, // token length = 0 (for Initial)
+        0x00, 0x04, // payload length = 4
+        0x00, 0x00, 0x00, 0x00, // dummy payload
+    )
+
+    data class PingResult(
+        val rttMs: Int,
+        val reachable: Boolean,
+    )
+
+    /**
+     * Ping a relay server via UDP. Returns RTT in ms, or unreachable.
+     * Thread-safe, can be called from coroutine on Dispatchers.IO.
+     */
+    fun ping(address: String): PingResult {
+        return try {
+            val parts = address.split(":")
+            if (parts.size != 2) return PingResult(0, false)
+            val host = parts[0]
+            val port = parts[1].toIntOrNull() ?: return PingResult(0, false)
+
+            val socket = DatagramSocket()
+            socket.soTimeout = TIMEOUT_MS
+            val dest = InetSocketAddress(host, port)
+
+            val sendPacket = DatagramPacket(PROBE, PROBE.size, dest)
+            val recvBuf = ByteArray(1200)
+            val recvPacket = DatagramPacket(recvBuf, recvBuf.size)
+
+            val start = System.nanoTime()
+            socket.send(sendPacket)
+            socket.receive(recvPacket) // blocks until response or timeout
+            val rttMs = ((System.nanoTime() - start) / 1_000_000).toInt()
+
+            socket.close()
+            PingResult(rttMs, true)
+        } catch (e: Exception) {
+            Log.w(TAG, "ping $address failed: ${e.message}")
+            PingResult(0, false)
+        }
+    }
+}