deterministic mirror ports derived from server MAC + upstream port

mirror = (upstream_port ^ (mac[0]<<8 | mac[5])); clamped to >=1024.
Same server+upstream always yields the same local port so the user
knows where to connect without checking the UI each time. Falls back
to OS-assigned port if the deterministic one is already in use.

win/gatuna-client/SessionManager.cs

@@ -88,15 +88,48 @@ sealed class SessionManager : IDisposable
         }
     }
 
+    /// <summary>
+    /// Compute a deterministic mirror port from the server MAC and the
+    /// upstream port. XOR the upstream port with (mac[0]<<8 | mac[5]),
+    /// then ensure the result is outside the privileged range.
+    /// </summary>
+    static ushort ComputeMirrorPort(byte[] serverMac, ushort upstreamPort)
+    {
+        var k = (ushort)((serverMac[0] << 8) | serverMac[5]);
+        var port = (ushort)(upstreamPort ^ k);
+        if (port < 1024)
+            port += 1024;
+        return port;
+    }
+
     /// <summary>
     /// Start a local TCP listener for the given upstream. Returns the mirror
     /// port, or 0 on failure.
     /// </summary>
     public int StartListener(UpstreamEntry upstream)
     {
-        var listener = new TcpListener(IPAddress.Loopback, 0);
+        if (_serverMac == null)
+            return 0;
+
+        var preferred = ComputeMirrorPort(_serverMac, upstream.Port);
+
+        // Try the deterministic port first; fall back to OS assignment.
+        TcpListener listener;
+        int port;
+        try
+        {
+            listener = new TcpListener(IPAddress.Loopback, preferred);
             listener.Start();
-        var port = ((IPEndPoint)listener.LocalEndpoint).Port;
+            port = ((IPEndPoint)listener.LocalEndpoint).Port;
+        }
+        catch
+        {
+            listener = new TcpListener(IPAddress.Loopback, 0);
+            listener.Start();
+            port = ((IPEndPoint)listener.LocalEndpoint).Port;
+            Log?.Invoke($"port {preferred} in use, fell back to {port}");
+        }
+
         var state = new ListenerState(listener, upstream);
         _listeners[port] = state;
         _ = AcceptLoop(state);