implement editor-above-Claude split in workspace

Editor opens above Claude via Ctrl+E with a draggable divider at
35% height (clamped 15-70%). Ctrl+W or Escape closes it. The
workspace no longer renders a tab bar — Claude is the always-visible
primary surface per D-047/D-048; the editor is a split overlay
per D-049, not a tab. The _WorkspaceSlot separates editor from
primary content tabs and renders them as a vertical split when the
editor is open.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/test/kernel/src/panels/arrangement_test.dart

@@ -100,5 +100,42 @@ void main() {
       a.applyPreset(classicPreset());
       expect(a.isInFocusMode, false);
     });
+
+    test('openEditor sets editorOpen', () {
+      final a = LayoutArrangement()..applyPreset(classicPreset());
+      expect(a.editorOpen, false);
+      a.openEditor();
+      expect(a.editorOpen, true);
+    });
+
+    test('closeEditor clears editorOpen', () {
+      final a = LayoutArrangement()..applyPreset(classicPreset());
+      a.openEditor();
+      a.closeEditor();
+      expect(a.editorOpen, false);
+    });
+
+    test('toggleEditor flips editorOpen', () {
+      final a = LayoutArrangement()..applyPreset(classicPreset());
+      a.toggleEditor();
+      expect(a.editorOpen, true);
+      a.toggleEditor();
+      expect(a.editorOpen, false);
+    });
+
+    test('setEditorRatio clamps to 0.15–0.70', () {
+      final a = LayoutArrangement()..applyPreset(classicPreset());
+      a.setEditorRatio(0.05);
+      expect(a.editorRatio, 0.15);
+      a.setEditorRatio(0.90);
+      expect(a.editorRatio, 0.70);
+      a.setEditorRatio(0.40);
+      expect(a.editorRatio, 0.40);
+    });
+
+    test('editorRatio defaults to 0.35', () {
+      final a = LayoutArrangement()..applyPreset(classicPreset());
+      expect(a.editorRatio, 0.35);
+    });
   });
 }