Performance: frame broadcasts once, Socket.Select instead of fixed sleep, reuse message list

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Protocol.cs

@@ -101,6 +101,7 @@ public sealed class Connection
     DateTime closeDeadline;
     string closingReason;
     readonly byte[] readBuffer = new byte[64 * 1024];
+    readonly List<Message> received = new();
     byte[] buffer = new byte[64 * 1024];
     int count;
 
@@ -113,9 +114,16 @@ public sealed class Connection
         RemoteAddress = client.Client.RemoteEndPoint?.ToString() ?? "?";
     }
 
+    /// <summary>Socket zum Warten auf Daten (Socket.Select im Server-Loop).</summary>
+    public Socket Socket => client.Client;
+
+    /// <summary>
+    /// Neue Nachrichten. Die Liste wird beim nächsten Aufruf wiederverwendet (keine Allokation pro Durchlauf).
+    /// </summary>
     public List<Message> Poll()
     {
-        var messages = new List<Message>();
+        var messages = received;
+        messages.Clear();
         if (IsClosed) return messages;
         if (closing)
         {
@@ -124,12 +132,13 @@ public sealed class Connection
         }
         try
         {
-            if (client.Client.Poll(0, SelectMode.SelectRead) && client.Available == 0)
+            if (client.Available == 0)
             {
-                Close("Connection closed");
+                // Getrennt? "lesbar, aber keine Daten" = Gegenseite hat geschlossen
+                if (client.Client.Poll(0, SelectMode.SelectRead) && client.Available == 0) Close("Connection closed");
                 return messages;
             }
-            while (stream.DataAvailable)
+            while (client.Available > 0)
             {
                 int read = stream.Read(readBuffer, 0, readBuffer.Length);
                 if (read <= 0) break;
@@ -145,12 +154,14 @@ public sealed class Connection
         return messages;
     }
 
-    public void Send(MsgType type, byte[] payload = null)
+    public void Send(MsgType type, byte[] payload = null) => SendFrame(Wire.Frame(type, payload));
+
+    /// <summary>Fertig gerahmte Nachricht senden (Broadcast: einmal rahmen, an alle schicken).</summary>
+    public void SendFrame(byte[] frame)
     {
         if (IsClosed || closing) return;
         try
         {
-            byte[] frame = Wire.Frame(type, payload);
             stream.Write(frame, 0, frame.Length);
         }
         catch (Exception e)