Implement Riverpod TabStateManager with in-memory snapshots

Prefer `AsyncNotifier` over `StateNotifier` because initialization requires async work (repository read). This gives idiomatic loading/error state for free. Define `TabState` as a `freezed` or manually-implemented immutable class with a `copyWith` method. Use a `List.unmodifiable` for the snapshots list to prevent accidental mutation.

For the debounce pattern, use a `Timer? _debounceTimer` field in the notifier — cancel and restart on each mutation, flush on dispose. The `NavigationStateRepository` interface should be injected via a Riverpod `Provider` that is overridden in tests with a `FakeNavigationStateRepository`. Avoid calling `GoRouter` from the notifier — the notifier only manages state; GoRouter observes the notifier via a listener in the shell widget.

This separation is critical for testability.

Unit tests in `test/navigation/tab_state_manager_test.dart` using `ProviderContainer`: (1) Initialize with a mock repository returning pre-seeded snapshots — assert `state.snapshots` matches. (2) Call `setActiveTab(2)` — assert `state.activeTabIndex == 2` and repository write was called. (3) Call `setActiveTab(5)` — assert `RangeError` is thrown. (4) Call `updateStackSnapshot(1, mockSnapshot)` — assert `state.snapshots[1] == mockSnapshot`.

(5) Call `getSnapshot(3)` — assert returns snapshot at index 3. (6) Simulate repository write failure — assert state was still updated in-memory and error was logged. (7) Rapid-fire 10 `setActiveTab` calls — assert repository write called at most twice (debounce). Target: 90%+ line coverage on notifier class.