Follow the standard MQTT Discovery split: discovery config stays under the HA
discovery prefix (homeassistant/.../config, required), but the actual state and
availability are published under our own prefix (default home_automation/). The
config payload's state_topic/availability fields point at the new prefix so HA
reads state from there.
- new config ha_state_topic_prefix (default home_automation).
- build_discovery_payload + all publish_* split discovery_prefix (config topic)
from state_prefix (state/availability). energy-cost still omits availability;
modbus availability structure unchanged. Tests updated.
tliu93