type InitialDataFn<Data extends object = object> = (payload: InitialDispatchContract) => Data | Promise<Data>;
Factory form of initialData in new State({ name, initialData }) — receives the
resolved signal context ( InitialDispatchContract) and returns the default
state value, sync or async. Runs on every access for a signal that has no persisted
value yet, so the default can be derived from the actual entry.