Implement async record count fetching in BLoC

Use flutter_bloc's EventTransformer for debouncing: apply `restartable()` or `droppable()` from bloc_concurrency package on the FetchRecordCount handler registration — `on(_onFetchRecordCount, transformer: restartable())`. This handles both debounce and cancellation elegantly without manual stream subscriptions. The `restartable()` transformer cancels the previous handler coroutine when a new event arrives. Add a 400ms artificial debounce inside the handler using `await Future.delayed(const Duration(milliseconds: 400))` before calling the service — with `restartable()`, this effectively debounces the service call.

Compute `showIncompleteWarning` by comparing `selectedRange.end` against a `lastCompleteReportingPeriodEnd` value that should be available from PeriodPresetService or a config constant. Emit state updates using `copyWith` on the existing loaded state to preserve all other fields.

Unit tests with bloc_test and fake_async or rxdart test utilities. Required test cases: (1) debounce — dispatch FetchRecordCount 3 times within 100ms, advance fake timer 400ms, verify service called exactly once; (2) cancellation — dispatch FetchRecordCount, advance 200ms, dispatch again, advance 400ms, verify only the second result is emitted; (3) isCountLoading transitions — verify isCountLoading: true emitted before service resolves, isCountLoading: false after; (4) service error — verify recordCount: null and isCountLoading: false emitted, not a PeriodSelectionError full state; (5) showIncompleteWarning — provide a range ending tomorrow, verify warning flag is true in emitted state; (6) SelectPreset trigger — dispatch SelectPreset, verify FetchRecordCount is subsequently processed. Use a MockPeriodRecordCountService that is async-controllable via Completer.