add status-bar collapse toggles for sidebar + context pane (T-294)

Fixed ~24px caret-line cells bookend the status bar (left=sidebar, right=context);
chevron flips per arrangement.isCollapsed (inward=collapse, outward=expand) and
fires the existing sidebar.collapse / context.collapse commands — the mouse
affordance for the already keyboard/CLI-addressable action (D-6). Visual only;
no new collapse logic. Status items shift 28px inward for the reserved cells.

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

test/app_collapse_toggle_test.dart

@@ -0,0 +1,91 @@
+/// Status-bar collapse toggles (T-294): a fixed cell bookends each end of the
+/// status bar, flips a caret-line chevron per arrangement.isCollapsed, and fires
+/// the existing sidebar.collapse / context.collapse commands.
+library;
+
+import 'package:clide/app.dart';
+import 'package:clide/builtin/default_layout/default_layout.dart';
+import 'package:clide/extension/extension.dart';
+import 'package:clide/kernel/kernel.dart';
+import 'package:clide/widgets/widgets.dart';
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import 'helpers/kernel_fixture.dart';
+
+Finder _icon(PhosphorIconPainter p) => find.byWidgetPredicate((w) => w is ClideIcon && w.painter == p);
+
+Widget _bar(KernelFixture f) => Directionality(
+      textDirection: TextDirection.ltr,
+      child: ClideKernel(
+        services: f.services,
+        child: ClideTheme(
+          controller: f.services.theme,
+          child: const MediaQuery(
+            data: MediaQueryData(size: Size(800, 200)),
+            child: Align(
+              alignment: Alignment.topLeft,
+              child: SizedBox(width: 800, height: 26, child: StatusbarHost()),
+            ),
+          ),
+        ),
+      ),
+    );
+
+void main() {
+  late KernelFixture f;
+
+  setUp(() async {
+    f = await KernelFixture.create();
+    f.services.arrangement.applyPreset(const LayoutPresetContribution(
+      id: 'test',
+      displayName: 'test',
+      slots: [
+        LayoutSlot(slot: Slots.sidebar, position: SlotPosition.left, visible: true),
+        LayoutSlot(slot: Slots.contextPanel, position: SlotPosition.right, visible: true),
+      ],
+    ));
+    f.services.extensions.register(DefaultLayoutExtension());
+    await f.services.extensions.activate('builtin.default-layout');
+  });
+  tearDown(() => f.dispose());
+
+  testWidgets('both toggles show, chevrons point inward when panes are open', (tester) async {
+    await tester.pumpWidget(_bar(f));
+    await tester.pump();
+    // Sidebar (left end) collapses leftward → caret-line-left; context (right
+    // end) collapses rightward → caret-line-right.
+    expect(_icon(PhosphorIcons.caretLineLeft), findsOneWidget);
+    expect(_icon(PhosphorIcons.caretLineRight), findsOneWidget);
+  });
+
+  testWidgets('a collapsed pane flips its chevron outward (expand affordance)', (tester) async {
+    f.services.arrangement.setCollapsed(Slots.sidebar, true);
+    await tester.pumpWidget(_bar(f));
+    await tester.pump();
+    // Sidebar now collapsed → its chevron points right (expand); context still
+    // open → right. So two right-pointing, none left.
+    expect(_icon(PhosphorIcons.caretLineRight), findsNWidgets(2));
+    expect(_icon(PhosphorIcons.caretLineLeft), findsNothing);
+  });
+
+  testWidgets('tapping the sidebar toggle fires sidebar.collapse', (tester) async {
+    await tester.pumpWidget(_bar(f));
+    await tester.pump();
+    expect(f.services.arrangement.isCollapsed(Slots.sidebar), isFalse);
+
+    await tester.tap(_icon(PhosphorIcons.caretLineLeft)); // the open-sidebar toggle
+    await tester.pump();
+
+    expect(f.services.arrangement.isCollapsed(Slots.sidebar), isTrue);
+  });
+
+  testWidgets('a hidden pane reserves the cell but shows no toggle', (tester) async {
+    f.services.arrangement.setVisible(Slots.contextPanel, false);
+    await tester.pumpWidget(_bar(f));
+    await tester.pump();
+    // Sidebar toggle present (open → left); context hidden → no right chevron.
+    expect(_icon(PhosphorIcons.caretLineLeft), findsOneWidget);
+    expect(_icon(PhosphorIcons.caretLineRight), findsNothing);
+  });
+}

test/app_statusbar_test.dart

@@ -69,9 +69,11 @@ void main() {
     for (final width in [600.0, 3440.0]) {
       await pumpAt(tester, width);
       expect(tester.takeException(), isNull, reason: 'width=$width');
-      // StatusbarHost pads 8px each side → right item's right edge ≈ width - 8.
-      expect(tester.getTopRight(find.text('RIGHT')).dx, closeTo(width - 8, 1.0), reason: 'right group not at edge at width=$width');
-      expect(tester.getTopLeft(find.text('LEFT')).dx, closeTo(8, 1.0), reason: 'left not at start at width=$width');
+      // StatusbarHost pads 4px each side and reserves a fixed 24px collapse-
+      // toggle cell at each end (T-294), so status items sit 28px inside both
+      // edges: right item's right edge ≈ width - 28, left item's left ≈ 28.
+      expect(tester.getTopRight(find.text('RIGHT')).dx, closeTo(width - 28, 1.0), reason: 'right group not at edge at width=$width');
+      expect(tester.getTopLeft(find.text('LEFT')).dx, closeTo(28, 1.0), reason: 'left not at start at width=$width');
     }
   });
 
@@ -83,6 +85,7 @@ void main() {
     ));
     await pumpAt(tester, 3440.0);
     expect(tester.takeException(), isNull);
-    expect(tester.getTopRight(find.text('R2')).dx, closeTo(3440 - 8, 1.0));
+    // 4px pad + 24px reserved toggle cell at the right end (T-294).
+    expect(tester.getTopRight(find.text('R2')).dx, closeTo(3440 - 28, 1.0));
   });
 }