Add additive MQTT 5 protocol support

[Copilot]

The client only supported MQTT 3.x, while its generic message processor and persistence contracts could not model MQTT 5 properties, acknowledgements, authentication, negotiation, or session flows. This change adds MQTT 5 without changing the default protocol or legacy basic APIs.

• Protocol architecture

  • Add strict binary primitives, incremental framing, typed packets, ordered properties, and MQTT 5 codecs for CONNECT through AUTH.
  • Validate flags, reason codes, property placement, UTF-8, topics, filters, packet identifiers, and negotiated packet limits.

• Public API

  • Add MQTT_5_0, immutable MQTT 5 options/results, typed callbacks, and a separate advanced capability interface.
  • Preserve existing constructors, callbacks, MQTT 3.x behavior, and simple operations.

$client = new MqttClient(
    'localhost',
    1883,
    'client-id',
    MqttClient::MQTT_5_0,
);

$client->connect();
$client->publish('events/example', 'payload');

• Runtime behavior

  • Support negotiated capabilities, topic aliases, enhanced authentication, session expiry, reconnect/resume behavior, and server DISCONNECT events.
  • Add explicit QoS stages, receive-maximum flow control, queued publications, and message expiry.
  • Extend repository persistence with a compatibility adapter for legacy implementations.

• Safety and conformance

  • Distinguish malformed packets from protocol errors and redact sensitive wire data from logs.
  • Add packet vectors, malformed-input and state-machine coverage, MQTT 3.x regressions, broker interoperability tests, compliance traceability, migration guidance, and security documentation.

• CI

  • Replace privileged pull-request execution with least-privilege jobs.
  • Split quality, unit, integration, coverage, cross-broker, and pinned Paho conformance workflows.