test(studio): lock sidebar destination Y-stability on rail expansion

Adds a widget test asserting each destination icon keeps its Y position
whether the rail is collapsed or expanded — a recurring regression. To
drive expansion without a hover gesture (which trips RenderFlex overflow
mid-transition), a test-only startSidebarExpanded flag threads
StudioApp -> StudioShell -> _Sidebar (controller starts at 1.0, hover
disabled); _SidebarItems get stable ValueKeys. analyze clean; new test
and the existing smoke test pass.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

test/sidebar_test.dart

@@ -0,0 +1,77 @@
+// Locks the sidebar invariant: destination icons sit at the same Y
+// whether the rail is collapsed or expanded. The header rows above the
+// destinations list have FIXED pixel heights (see _SidebarState), so
+// expanding the rail (a width-only change) must not shift any
+// destination vertically. This was a recurring regression source; this
+// test pins it.
+//
+// Expansion is driven via StudioApp(startSidebarExpanded: true), NOT a
+// hover gesture — hover mid-transition trips RenderFlex overflow in
+// widget tests (see widget_test.dart). With the flag the rail's
+// animation controller starts at value 1.0 (fully expanded), so no
+// animation needs to settle.
+
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:chain_studio/data/hub.dart';
+import 'package:chain_studio/main.dart';
+
+const _destinations = <String>[
+  'welcome',
+  'store',
+  'doctor',
+  'flows',
+  'audit',
+  'approvals',
+  'federation',
+];
+
+Map<String, double> _destinationYs(WidgetTester tester) {
+  final ys = <String, double>{};
+  for (final id in _destinations) {
+    final finder = find.byKey(ValueKey('sidebar-item-$id'));
+    expect(finder, findsOneWidget, reason: 'destination "$id" must render');
+    ys[id] = tester.getTopLeft(finder).dy;
+  }
+  return ys;
+}
+
+void main() {
+  testWidgets(
+    'sidebar destination Y positions are stable across rail expansion',
+    (tester) async {
+      // Collapsed (default).
+      await tester.pumpWidget(
+        const StudioApp(
+          initialThemeMode: ThemeModeValue.system,
+          initialLocale: Locale('en'),
+        ),
+      );
+      // pump (not pumpAndSettle): the app has long-lived timers/animations
+      // that never "settle"; one frame is enough to lay the sidebar out.
+      await tester.pump(const Duration(milliseconds: 100));
+      final collapsed = _destinationYs(tester);
+
+      // Expanded: controller starts at 1.0 via the flag, hover disabled.
+      await tester.pumpWidget(
+        const StudioApp(
+          initialThemeMode: ThemeModeValue.system,
+          initialLocale: Locale('en'),
+          startSidebarExpanded: true,
+        ),
+      );
+      await tester.pump(const Duration(milliseconds: 100));
+      final expanded = _destinationYs(tester);
+
+      for (final id in _destinations) {
+        expect(
+          expanded[id],
+          closeTo(collapsed[id]!, 0.5),
+          reason: 'destination "$id" Y shifted on expansion '
+              '(${collapsed[id]} -> ${expanded[id]}) — the rows above the '
+              'destinations list must keep fixed heights',
+        );
+      }
+    },
+  );
+}