2e2ffee41a
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)