ConstReadonlyLevel: ControlKindWatch a falling level and warn before it reaches empty.
ReadonlyMonitor: ControlKindReport readings only, with no output and no alerts.
ReadonlySetpoint: ControlKindHold a reading near a setpoint by switching an output on and off.
ReadonlySurge: ControlKindWarn when a reading changes faster than a limit.
Which control policy a profile applies to each reading.
Provided as a runtime object plus a matching string-union type.