Design a Package Tracking System with Courier Assignment, Event Recording, Undo, and Sign-Out

Implement a multi-level package tracking system supporting: recording parcel events under an assigned courier (RecordCourierEvent), querying total event counts per courier across all parcels (GetCourierEventCount), undoing the most recent courier assignment change for a parcel (UndoCourierChange), and signing out couriers with appropriate state transitions — signed-out couriers cannot be assigned or record events, their parcels become unassigned, and the unassignment does not create an undo-history entry.