ultrawide layout test coverage + shared setSurfaceSize helper (T-241)

Add setSurfaceSize(tester, width) to the widget harness — the reusable way to
test width-sensitive layout at an ultrawide surface (a wide SizedBox under the
default 800px surface is clamped, so the view's physicalSize must be set), the
foundation T-241 asked for.

Ultrawide cases on the surfaces most prone to width-proportional bugs (the T-239
class): ClideMarquee stays static when a line fits a 3440 slot (it was only
tested narrow); the quick-open palette stays width-capped, not stretched edge to
edge. The status bar already covers 600+3440 (T-239).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/widgets/quick_open_overlay_test.dart

@@ -163,4 +163,15 @@ void main() {
     await pumpAsync(tester);
     expect(find.text('No matching files'), findsOneWidget);
   });
+
+  testWidgets('the palette is width-capped (not stretched) on an ultrawide surface (T-241)', (tester) async {
+    setSurfaceSize(tester, 3440);
+    await tester.pumpWidget(harness(f, const QuickOpenOverlay()));
+    f.services.quickOpen.open();
+    await pumpAsync(tester);
+    // The panel is a fixed 480 — the filter field must stay capped, not span the
+    // full 3440 (a Row/Expanded regression would stretch it edge to edge).
+    final field = tester.getRect(find.byType(EditableText));
+    expect(field.width, lessThan(600), reason: 'capped to the panel, not stretched across the ultrawide surface');
+  });
 }