Publish retained per-zone MQTT states with optional Home Assistant discovery

[bharat]

Implements the optional phase from the MQTT panel events design doc: zone state parity with HTTP, without polling.

When --mqtt-zones (or [mqtt] zones = true) is set, every ZONE_STATUS / ZONE_DATA message publishes a retained per-zone state to {prefix}/zone/{number}/state. The payload mirrors the /zones REST field names (partition, area, group, number, name, state, type) and adds a tripped boolean, so MQTT consumers do not need to reimplement state-list parsing. The bootstrap zone scan publishes all zones at startup, and retained delivery means late subscribers (or a restarting Home Assistant) see current state immediately.

With --mqtt-discovery-prefix homeassistant (or [mqtt] discovery_prefix), a retained Home Assistant MQTT discovery config is also published once per zone, so zone binary sensors appear in HA automatically, grouped under a Concord232 device, with availability bound to {prefix}/status. A last-will message now marks {prefix}/status offline (retained) so availability tracks reality when the server dies.

Both features are opt-in and default off; existing MQTT behavior (ALARM / TOUCHPAD events, online status) is unchanged apart from the added LWT. Zone publishing is decoupled from the panel interface via a small handler factory so it stays unit-testable; 5 new tests cover payload shape, retained publishing, one-time discovery, the disabled path, and merged-record lookup.

Context: we run this server against a Concord 4 with Home Assistant consuming /zones at a 1 second poll. This lets us retire that polling entirely.

Design doc table updated to reflect the implemented topic shape.