perf: Take WM_COPYDATA off the UI thread and out of the WndProc

src/D2BotNG/Program.cs

@@ -124,7 +124,9 @@ private static void Main(string[] args)
             // EngineHostedService.StartAsync (and any handoff RehydrateAsync inside it) reads
             // MessageWindow.Handle, so it must be valid by then. In GUI mode, MainForm no
             // longer switches the handle when it loads — the message-only window owns it
-            // for the full process lifetime.
+            // for the full process lifetime. The window runs on its own pump thread, so this
+            // returns once the handle exists and nothing here shares a pump with it: senders
+            // block on SendMessageW, and the UI thread below is about to host WebView2.
             var messageWindow = app.Services.GetRequiredService<MessageWindow>();
             messageWindow.CreateMessageOnlyWindow();
 

src/D2BotNG/Services/D2BSMessageHandler.cs

@@ -77,8 +77,24 @@ protected override async Task ExecuteAsync(CancellationToken stoppingToken)
     {
         _logger.LogInformation("D2BS message handler started");
 
-        await foreach (var msg in _messageWindow.Messages.ReadAllAsync(stoppingToken))
+        await foreach (var raw in _messageWindow.Messages.ReadAllAsync(stoppingToken))
         {
+            // Decoding and JSON parsing happen here rather than in the WndProc: the sending game
+            // is blocked in SendMessageW for the whole of that call, and the rest of the fleet is
+            // queued behind it. Parse returns null for a payload it cannot read (already logged),
+            // and the buffer goes back to the pool either way.
+            D2BSMessage? msg;
+            try
+            {
+                msg = _messageWindow.Parse(raw);
+            }
+            finally
+            {
+                raw.Release();
+            }
+
+            if (msg == null) continue;
+
             try
             {
                 await HandleMessageAsync(msg);