fix(layout): barre supérieure CLI custom cliquable — stacking context du corps de leaf

Le corps d'une leaf (layout-leaf-body) crée désormais son propre stacking context (z-index 0). Les vues CLI custom peuvent comporter des enfants à haut z local (toolbar z-20) qui restaient au-dessus des contrôles de cellule (CELL_Z 5) et captaient les clics ; ils restent désormais sous les contrôles. Test de layering dédié ajouté dans LayoutGrid.cellControlsLayering.test.tsx.

frontend/src/features/layout/LayoutGrid.tsx

@@ -338,6 +338,10 @@ function goToCell(nodeId: string): void {
  *   full-cell veils   4   (write-portal + F3 overlays; mutually exclusive)
  *   cell controls     5   (always on top and clickable)
  *   turn actions      6   (busy-state actions; Cancel must stay visible)
+ *
+ * The terminal/chat body must create its own z-index 0 stacking context. Custom
+ * CLI views can legitimately contain local high-z children (for example their
+ * own toolbar), but those children must stay below the cell controls.
  */
 const CELL_Z = {
   banner: 3,
@@ -1300,8 +1304,10 @@ function LeafView({
           navigating. The agent cell passes its {@link WritePortal} so keystroke
           counting + injection suspension are wired; a plain cell passes none. */}
       <div
+        data-testid="layout-leaf-body"
         style={{
           position: "relative",
+          zIndex: 0,
           width: "100%",
           height: "100%",
           minHeight: 0,