[Feature] Replace MqttActor cmd-kind dispatch with match()

[pathosDev]

Size / Priority

• Size: S (~40 lines)
• Category: C.1 Pattern-Matching.
• Risk: low.

Affected files

• src/io/broker/MqttActor.ts:240-251 — onReceive's cmd-kind dispatch.

Background

MqttActor is the MQTT 5.0 broker actor. Its command dispatch handles:

• 'publish' — emit a PUBLISH packet.
• 'subscribe' — emit a SUBSCRIBE packet.
• 'unsubscribe' — emit UNSUBSCRIBE.
• (potentially more, including user-property variants — verify during implementation).

Same pattern as #232 (JetStream): an if/else chain over discriminator strings. Adding a new cmd silently falls through without compile error.

Target code

import { match } from 'ts-pattern';

override onReceive(cmd: MqttCmd): void {
  match(cmd)
    .with({ kind: 'publish' },     (c) => this.handlePublish(c))
    .with({ kind: 'subscribe' },   (c) => this.handleSubscribe(c))
    .with({ kind: 'unsubscribe' }, (c) => this.handleUnsubscribe(c))
    .exhaustive();
}

Integration / risk

• No behavioural change.
• Per-handler typing improves (narrowing from match).
• MQTT user-properties (MQTT 5.0 user properties + reason codes in MqttActor #13, landed in v0.7.0) flow through unchanged.

Test plan

1. Regression — MQTT 5.0 test suite passes.
2. Compile-time — new variant in the union produces error.
3. MQTT integration — message round-trip through the broker still works.

Acceptance criteria

• onReceive uses match().exhaustive().
• Tests pass.
• No behaviour drift on MQTT 5.0 user-properties / reason-codes.
• No CHANGELOG entry needed.