Wire recalculation trigger on selection change

In flutter_bloc, the cleanest way to implement cancellation of in-flight async work is to use a Completer or store a reference to the current calculation Future and check whether the event ID has changed before emitting. Alternatively, use EventTransformer with rxdart's switchMap: `transformer: (events, mapper) => events.switchMap(mapper)`. This is the idiomatic BLoC approach for cancellable async operations. The debounce can be added on top: `transformer: (events, mapper) => events.debounceTime(const Duration(milliseconds: 300)).switchMap(mapper)`.

Inject the transformer via the on registration. Ensure that the ExpenseCalculationResult is a sealed/immutable value object — never mutate the result after emission. The 'loading calculation' state should still carry the full selectedIds and disabledTypeIds so the UI can show a spinner without hiding the already-made selections.

Unit tests with flutter_test and bloc_test. Use a fake ExpenseCalculationService that supports configurable async delay and error injection. Test cases: (1) single toggle → calculating state → loaded state with result, (2) two rapid toggles → only the second result applied (first cancelled), (3) calculation error → error state with selection preserved, (4) recovery: successful calculation after an error. Use fakeAsync and pump to control time in tests so the debounce window can be tested deterministically without real delays.