Parcel Tracking System

Implement a progressively unlocked parcel-tracking system. The first level records per-parcel event counts; later levels add parcel ranking, courier assignment with courier-specific events, and undo plus courier sign-out behavior. Requirements Implement a simplified parcel-tracking system in the supplied class structure. Progress through multiple levels; each level unlocks after the current unit tests pass. Level 1 — parcel event counts Each parcel has a string parcelId. Counts are maintained independently for each string eventType. int? RecordEvent(string parcelId, string eventType, int count) Add count to the running total for the parcel and event type. Create the event type when it does not yet exist. Return the new running total. int? GetEventCount(string parcelId, string eventType) Return the current running count. Return null when either the parcel or event type does not exist. bool RemoveEvent(string parcelId, string eventType) Remove the event-type record from the parcel. Return true when a record was removed and false when it did not exist. Delete the parcel when removing the event leaves it with no remaining event types. Later levels Level 2: rank parcels by the number of recorded events. Level 3: support courier assignment and courier-specific event recording. Level 4: support undoing courier changes and courier sign-out. Examples RecordEvent("A", "B", 5) -> 5 RecordEvent("A", "B", 6) -> 11 GetEventCount("A", "B") -> 11 GetEventCount("A", "C") -> null RemoveEvent("A", "B") -> true RemoveEvent("A", "B") -> false Notes The environment is unit-test driven; passing behavior matters more than producing the most efficient implementation. A single test can be run from the terminal with the supplied run_single_test.sh command. Data from the current and all previous levels remains available, so later operations should reuse earlier behavior instead of duplicating it. The first-level interface is complete; the exact later-level method signatures are not exposed. Preparation Implement the Level 1 interface and unit-test creation, accumulation, missing lookups, repeated removal, and automatic parcel cleanup. Add a deterministic ranking layer over the same state without breaking Level 1 behavior. Sketch explicit courier-assignment and undo state transitions, including sign-out after reassignment. Practice evolving one class through progressive hidden-test levels while keeping earlier contracts stable.