Demand State Id
Type-safe identifier for the lifecycle states of a SupplyChainModel.Demand. Each state in the demand state machine has exactly one DemandStateId singleton; the sealed-class hierarchy lets listener bodies use exhaustive when and referential identity (===) checks instead of stringly-typed comparison on displayName.
The state-machine implementation lives on SupplyChainModel.DemandState and uses these IDs to label each concrete state class.