MQTT support with Home Assistant discovery (optional)

[sethpjohnson]

Summary

Stacked on #5 (→ #4 → #3 → #2 → #1) — the sixth and final PR in the stack; merge in order. Off by default: setting nothing changes existing behavior.

Publishes controller status to MQTT and accepts commands, so the R730's fan state appears in Home Assistant as a self-registered device with full control parity.

• Decoupled bridge (internal/mqtt, paho) consuming the same controller methods the HTTP API uses — so MQTT commands inherit every safety rule from the earlier tiers: min/max clamping, 24h override cap, critical-temp ramp override, hint validation. The controller package stays MQTT-unaware.
• A flaky broker can't touch cooling: connect is fire-and-forget, publish/subscribe/stop are bounded (2s), and bridge teardown is sequenced after the Tier 1 BMC restore with a 3s bound. Verified live: SIGINT shows restore → MQTT teardown → exit in ~11ms.
• HA Discovery: one device, 11 entities (6 sensors, 3 binary sensors, an override-speed number bounded to your configured min/max, a clear-override button), retained and republished on reconnect.
• Per-tick retained state + Last Will: HA marks the device unavailable the instant the process dies — a free monitor for the fail-safe scenarios.
• Shared validation: internal/validate extracted from the HTTP layer (verified byte-for-byte lossless) so HTTP and MQTT enforce identical rules. Retained command messages are dropped (logged once/topic) so a stray mosquitto_pub -r can't replay a stale command on every restart.

Review & verification

• Principal review → APPROVED WITH SUGGESTIONS, then the one High-Priority item (retained-command replay) fixed pre-landing and APPROVED on a confirmation re-review (reviewer traced paho source to confirm the Retain bit is independent of Dup — the drop can never false-positive on a live command).
• go vet + go test -race clean across all 8 packages; integration test drives a real paho client against a real in-process mochi broker (connect→discovery→command→state→LWT + retained-drop), stable across repeated runs.
• Acceptance: PASS 9/9 live against a running broker — including a 100→80 clamped command round-trip, retained-drop across a restart, LWT on SIGKILL, and restore-before-teardown shutdown ordering.
• Follow-ups filed: dex f5u9g234 (topic charset validation, shared WorkloadHint constructor, Subscribe-timeout retry).

⚠️ Security note (also in the README): MQTT command authority = broker access. Anyone who can publish to your broker can command the fans — use broker credentials/ACLs. Blast radius stays bounded by the controller-level clamps and the critical-temp override. No TLS in v1.

Task: dex e6fla2yl — MQTT / Home Assistant support

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