Write unit tests for NavigationStateRepository

The test file should be placed at test/navigation/navigation_state_repository_test.dart. Since SharedPreferences is injected into the repository constructor, no mockito mocks are needed — just use the fake instance from setMockInitialValues. For the 5-tab loop test, use a for loop with test('tab $i round-trip', ...) inside a group to avoid code duplication. For the corrupted data graceful-handling tests, pre-seed the raw SharedPreferences key directly (bypassing the repository's write path) to simulate real-world corruption scenarios.

Ensure tearDown calls SharedPreferences.setMockInitialValues({}) even if test throws, or rely on setUp being called before every test. Keep fixtures (sample route stacks) in a local const or a test fixture file shared with task-001 model tests.

Use SharedPreferences.setMockInitialValues({}) in a setUp() block to guarantee a clean store before each test. Instantiate NavigationStateRepository by passing the fake SharedPreferences.getInstance() result obtained after setMockInitialValues. Write one test group per method: 'saveTabIndex / loadTabIndex', 'saveStackSnapshot / loadStackSnapshot', 'clearAll', 'corrupted data handling'. For corrupted data tests, use SharedPreferences.setMockInitialValues({'nav_state.tab_index': 'not-a-number'}) to pre-seed invalid data.

Use expect(result, isNull) assertions — never expect an exception for graceful degradation paths. Run flutter test --coverage and confirm branch coverage with a coverage report.