MQTT publisher::spawn is never callled.

[Yesuah]

MQTT publisher is fully implemented but publisher::spawn is never called from main.rs. args.mqtt is never read. Fix adds the channel, wires broadcast_tick_task, and spawns the publisher on startup.

[ruvnet]

Thanks for catching this — your diagnosis was correct at the time this PR was opened: the MQTT publisher was implemented but not spawned.

Triage update: this is now resolved on main via the #872 work, implemented with a slightly different (typed) shape, so this PR is superseded:

• main.rs:6178 — if args.mqtt_opts.mqtt { … } reads the flag (the arg moved into a grouped mqtt_opts).
• main.rs:6182 — mqtt::config::MqttConfig::from_args(&args.mqtt_opts) + mcfg.validate().
• main.rs:6195 — let (vtx, vrx) = broadcast::channel::<mqtt::state::VitalsSnapshot>(64);
• main.rs:6196 — mqtt::publisher::spawn(mcfg, builder, vrx); ← the spawn you flagged.
• main.rs:6197+ — bridges the existing JSON sensing broadcast into the typed VitalsSnapshot stream the publisher consumes; gated behind #[cfg(feature = "mqtt")] (the Docker image builds --features mqtt).

Because of that, this PR's diff (which targets the older flat args.mqtt / args.mqtt_host fields) no longer applies cleanly — those fields are now under args.mqtt_opts, so it would need a rebase to even compile, and the behaviour it adds is already present.

Recommend closing as superseded by #872. Appreciate the clear write-up — it matched the root cause exactly.