add --verbose flag and TCP RST on connection failure

Server (gatunad):
- New --verbose/-v flag: logs lifecycle events at info level
  (DISCOVER, MANIFEST sent, OPEN, session established, CLOSE,
  OPEN_NAK). Without the flag, errors only as before.

Client (gatuna):
- OPEN_NAK and session CLOSE now send TCP RST to the local app
  instead of a graceful FIN. LingerOption(true, 0) causes Winsock
  to emit RST on close. The local app (e.g. ssh) sees a broken
  connection instead of a clean close, which is more honest about
  what happened (upstream was unreachable).

gatuna-win/SessionManager.cs

@@ -85,7 +85,7 @@ sealed class SessionManager : IDisposable
                 lock (_openLock)
                 {
                     if (_pending != null)
-                        _pending.Client.Dispose();
+                        NetUtil.RstClose(_pending.Client);
                 }
                 ProcessQueue();
                 break;
@@ -546,7 +546,21 @@ sealed class Session(
         _cts.Cancel();
         _deliverChannel.Writer.TryComplete();
         SendClose();
-        try { client.Dispose(); } catch { }
+        NetUtil.RstClose(client);
         _cts.Dispose();
     }
 }
+
+/// Close a TcpClient with a TCP RST instead of a FIN.
+static partial class NetUtil
+{
+    public static void RstClose(TcpClient c)
+    {
+        try
+        {
+            c.LingerState = new LingerOption(true, 0);
+            c.Close();
+        }
+        catch { }
+    }
+}