feat(mqtt): add MQTT packet parser, dispatch, and ReadLoop integration

Implement Task 2 of MQTT orchestration:
- Create MqttParser.cs with loop-based packet parser and dispatch switch
- Add MqttReader field to MqttHandler for per-connection parsing state
- Wire ReadLoop to call MqttParser for MQTT connections
- Implement PINGREQ handler (enqueues PINGRESP)
- CONNECT-first enforcement (rejects non-CONNECT as first packet)
- Partial packet handling via MqttReader pending buffer
- 13 unit tests covering parser, dispatch, partial packets, and edge cases
- Stub dispatch entries for CONNECT, PUBLISH, SUB, UNSUB, DISCONNECT (NotImplementedException)

dotnet/src/ZB.MOM.NatsNet.Server/ClientConnection.cs

@@ -25,6 +25,7 @@ using Microsoft.Extensions.Logging;
 using ZB.MOM.NatsNet.Server.Auth;
 using ZB.MOM.NatsNet.Server.Internal;
 using ZB.MOM.NatsNet.Server.Internal.DataStructures;
+using ZB.MOM.NatsNet.Server.Mqtt;
 using ZB.MOM.NatsNet.Server.Protocol;
 using ZB.MOM.NatsNet.Server.WebSocket;
 
@@ -1405,8 +1406,59 @@ public sealed partial class ClientConnection
     internal void ReadLoop(byte[]? pre)
     {
         LastIn = DateTime.UtcNow;
+
+        // Process any pre-read bytes first.
         if (pre is { Length: > 0 })
+        {
             TraceInOp("PRE", pre);
+            if (IsMqtt())
+            {
+                var preErr = MqttParser.Parse(this, pre, pre.Length);
+                if (preErr != null)
+                {
+                    CloseConnection(ClosedState.ParseError);
+                    return;
+                }
+            }
+        }
+
+        // MQTT clients use the MqttParser; NATS clients use ProtocolParser (not yet wired).
+        if (!IsMqtt())
+            return;
+
+        // Main read loop — read from network stream until closed.
+        var buf = new byte[32768]; // 32 KB read buffer
+        try
+        {
+            while (true)
+            {
+                int n;
+                try
+                {
+                    n = _nc!.Read(buf, 0, buf.Length);
+                }
+                catch
+                {
+                    break; // Connection closed or errored.
+                }
+
+                if (n <= 0)
+                    break; // Connection closed.
+
+                LastIn = DateTime.UtcNow;
+
+                var err = MqttParser.Parse(this, buf, n);
+                if (err != null)
+                {
+                    CloseConnection(ClosedState.ParseError);
+                    return;
+                }
+            }
+        }
+        finally
+        {
+            CloseConnection(ClosedState.ClientClosed);
+        }
     }
 
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