add network test feature (PING/PONG with latency stats)

New frame types PING (0x0B) and PONG (0x0C), each carrying an 8-byte
nonce. Server echoes PING nonce verbatim in PONG. Client sends pings
at random 10-100ms intervals, correlates nonces to measure RTT.

Stats shown live: sent/recv counts, loss %, average latency, jitter
(mean absolute delta of consecutive RTTs). Test button toggles on/off.

Updated both Rust server (echo in main.rs) and C# client (PingTest.cs,
SessionManager routing, MainForm Test button + stats label).

win/gatuna-client/SessionManager.cs

@@ -14,6 +14,7 @@ sealed class SessionManager : IDisposable
     byte[]? _serverMac;
     string _serverHostname = "";
     UpstreamEntry[] _upstreams = [];
+    PingTest? _pingTest;
 
     // Serialized OPEN: only one outstanding at a time.
     readonly object _openLock = new();
@@ -47,6 +48,22 @@ sealed class SessionManager : IDisposable
         _link.SendBroadcast(new Frame.Discover());
     }
 
+    public PingTest? StartPing()
+    {
+        if (_link == null || _serverMac == null)
+            return null;
+        _pingTest?.Stop();
+        _pingTest = new PingTest(_link, _serverMac);
+        _pingTest.Start();
+        return _pingTest;
+    }
+
+    public void StopPing()
+    {
+        _pingTest?.Stop();
+        _pingTest = null;
+    }
+
     public void HandleFrame(Frame frame, byte[] srcMac)
     {
         switch (frame)
@@ -85,6 +102,13 @@ sealed class SessionManager : IDisposable
                 if (_sessions.TryRemove(close.SessionId, out var s))
                     s.Dispose();
                 break;
+
+            case Frame.Pong pong:
+                _pingTest?.HandlePong(pong.Nonce);
+                break;
+
+            case Frame.Ping _:
+                break;
         }
     }
 
@@ -227,6 +251,7 @@ sealed class SessionManager : IDisposable
 
     public void StopAll()
     {
+        StopPing();
         foreach (var kv in _listeners)
             kv.Value.Listener.Stop();
         _listeners.Clear();