Support heat-pump/flexload nodes in Live Energy Balance and SoC-aware heat-curve control

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Roma is building a Heishamon driver and hit two current product gaps while testing on HA Green v0.121.0.

Current behavior:

• Flexloads can dispatch thermostat setpoints via setpoint_action and price/PV-aware planner/simple modes.
• The current flexload model can express min/max or target/min behavior, but not a discrete three-level heat-curve offset policy (-3, 0, +3) driven by both price and site battery SoC.
• Live Energy Balance currently builds nodes from /api/status driver fields pv_w, bat_w, ev_w, and grid/site meter. A heat pump/flexload that only emits metrics does not appear as its own node.

Desired behavior:

1. Allow a Heishamon/heat-pump flexload to expose/display as a heat-pump or flexible-load node in Live Energy Balance, distinct from EV and battery.
2. Consider a config-level policy for discrete flexload commands, e.g. set_heat_curve_offset values [-3, 0, +3] based on price bands and battery SoC / surplus state.
3. Avoid requiring driver authors to fake DerBattery or DerEV solely for UI visibility.

Notes:

• Existing valid host.emit types are meter, pv, battery, ev, vehicle.
• host.emit_metric is the right path for temperatures/diagnostics today, but metrics do not create a flow node.
• HeatSourceDriver is declared but not yet wired in flexload service, so central HP attribution may be related future work.