feat(FN-1470): implement ScreenRouter with tab bar and keyboard navigation

- Add ScreenRouter component for multi-screen navigation in TUI
- Implement tab bar with icon labels and active state indicators
- Support keyboard navigation with arrow keys and Tab/Shift+Tab
- Add mouse click support for tab selection
- Export new screen router components from package index
- Add comprehensive unit tests for navigation behavior
- Update documentation with architecture and gap analysis
- Update TUI README with ScreenRouter usage examples

packages/tui/src/components/index.ts

@@ -0,0 +1,16 @@
+/**
+ * @fusion/tui components
+ *
+ * Reusable UI components for the Fusion TUI.
+ */
+
+export {
+  ScreenRouter,
+  SCREENS,
+  getScreenById,
+  getScreenIndex,
+  type ScreenId,
+  type Screen,
+  type ScreenRouterProps,
+  type ScreenComponentProps,
+} from "./screen-router.js";