fix: premature socket teardown on CLOSE + duplicate CLOSE echo

Two bugs when upstream closes:

1. Data loss: gatuna received CLOSE from gatunad and immediately
   RST'd the socket to curl while the drain pump still had queued
   payloads in _deliverChannel. Fix: OnRemoteClose() completes the
   channel writer and cancels the read pump, but lets the drain pump
   finish naturally. Drain pump no longer takes _cts.Token — it stops
   on channel completion and closes the socket gracefully (FIN, OS
   flushes in background).

2. Duplicate CLOSE: gatuna echoed CLOSE back to gatunad (server
   already knows — it initiated). SendClose() now guarded by
   _closeSent flag. OnRemoteClose() never sends CLOSE at all.

Server-side: remove session from store before sending CLOSE so the
retransmit timer stops before CLOSE is queued. Only log CLOSE if
the session actually existed (dedup).

gatuna-win/SessionManager.cs

@@ -100,7 +100,7 @@ sealed class SessionManager : IDisposable
 
             case Frame.Close close:
                 if (_sessions.TryRemove(close.SessionId, out var s))
-                    s.Dispose();
+                    s.OnRemoteClose();
                 break;
 
             case Frame.Pong pong:
@@ -394,6 +394,7 @@ sealed class Session(
     readonly RecvState _recv = new();
     readonly object _sendLock = new();
     readonly object _recvLock = new();
+    volatile bool _closeSent;
 
     public void Start()
     {
@@ -403,6 +404,17 @@ sealed class Session(
             _ = RetransmitTimer();
     }
 
+    /// <summary>
+    /// Called when a CLOSE frame arrives from the server. The server has
+    /// already torn down its side — we just need to flush queued data to
+    /// the local socket and close gracefully. Do NOT echo CLOSE back.
+    /// </summary>
+    public void OnRemoteClose()
+    {
+        _cts.Cancel();
+        _deliverChannel.Writer.TryComplete();
+    }
+
     /// Handle a DATA frame from the tunnel.
     public void HandleData(Frame.Data data)
     {
@@ -440,10 +452,10 @@ sealed class Session(
 
     async Task PumpSocketToTunnel()
     {
+        var buf = new byte[Proto.MaxPayload];
         try
         {
             var stream = client.GetStream();
-            var buf = new byte[Proto.MaxPayload];
             using var reg = _cts.Token.Register(() => client.Dispose());
             while (!_cts.IsCancellationRequested)
             {
@@ -476,6 +488,8 @@ sealed class Session(
             }
         }
         catch { }
+        _cts.Cancel();
+        _deliverChannel.Writer.TryComplete();
         SendClose();
         onClosed();
     }
@@ -485,10 +499,11 @@ sealed class Session(
         try
         {
             var stream = client.GetStream();
-            await foreach (var payload in _deliverChannel.Reader.ReadAllAsync(_cts.Token))
-                await stream.WriteAsync(payload, _cts.Token);
+            await foreach (var payload in _deliverChannel.Reader.ReadAllAsync())
+                await stream.WriteAsync(payload);
         }
         catch { }
+        finally { client.Close(); }
     }
 
     async Task RetransmitTimer()
@@ -539,7 +554,12 @@ sealed class Session(
         link.SendTo(serverMac, new Frame.Data(sessionId, seq, ackSeq, []));
     }
 
-    void SendClose() => link.SendTo(serverMac, new Frame.Close(sessionId, null));
+    void SendClose()
+    {
+        if (_closeSent) return;
+        _closeSent = true;
+        link.SendTo(serverMac, new Frame.Close(sessionId, null));
+    }
 
     public void Dispose()
     {