Add MqttSessionStore with save/load/delete/list operations, flapper
detection (backoff on rapid reconnects), and TimeProvider-based testing.
Add MqttRetainedStore for per-topic retained messages with MQTT wildcard
matching (+/# filters). Add MqttQos2StateMachine tracking the full
PUBREC/PUBREL/PUBCOMP flow with duplicate rejection and timeout detection.
19 new tests: 9 session persistence, 10 QoS/retained message tests.
