ConstReadonlyConfirmed: SlotStateTried and confirmed working.
ReadonlyEmpty: SlotStateNothing has been written here.
ReadonlyFailed: SlotStateTried and did not confirm, so it will not be tried again.
ReadonlyPending: SlotStateBeing tried for the first time; it reverts unless it confirms.
ReadonlyReceiving: SlotStateAn image is arriving, and written says how much of it has.
ReadonlyStaged: SlotStateA complete image that matched its manifest, not yet tried.
What a device believes about one slot.
Provided as a runtime object plus a matching string-union type.