feat(ui): every app's frame around its tabs, Learn's moved into halkyon_ui

NavigationFrame is Learn's AppShell without its router and its tabs: a
navigation rail from medium windows on with the app's icon on top,
leading to the first tab, a bar floating above the foot below, the
background of the screen in view around it, and a tick on every choice.
TabsStack is Learn's container of the tabs' navigators, every tab kept
alive and told whether it shows. Learn's shell is built on both, its
tests unchanged; Train's is next.

The rail's icon is drawn from 120 px at most in memory, as Learn's was.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_ui/test/widget/tab_visibility_test.dart

@@ -61,4 +61,23 @@ void main() {
 
     expect(visible, isTrue);
   });
+
+  testWidgets('the tabs stack shows the tab chosen, keeps the others alive, '
+      'and tells each whether it shows', (tester) async {
+    final visible = <String, bool>{};
+    Widget tab(String name) => Builder(
+      builder: (context) {
+        visible[name] = TabVisibility.of(context);
+        return Text(name);
+      },
+    );
+    await tester.pumpWidget(
+      testApp(TabsStack(index: 1, children: [tab('Home'), tab('Maps')])),
+    );
+
+    expect(find.text('Maps'), findsOneWidget);
+    expect(find.text('Home'), findsNothing, reason: 'offstage');
+    expect(find.text('Home', skipOffstage: false), findsOneWidget);
+    expect(visible, {'Home': false, 'Maps': true});
+  });
 }