Editor: render a layer as one framed row of mark glyphs

A layer is one plotting space with several marks stacked in z-order, so it now renders as a single frame holding its child marks as a row of glyphs, badged with a new 'layers' glyph — instead of offset stacked cards that read as separate spaces and hid the lower glyphs. Each mark stays an individual treeitem, so selection and Alt+up/down z-order reorder are unchanged.

src/app/components/Icon.tsx

@@ -35,6 +35,7 @@ export type IconName =
   | 'info' // about / information — Carbon Information (outline)
   | 'revert' // revert draft to last published — Carbon Reset
   | 'structure' // composition-structure wireframe disclosure (preview toolbar) — nested view blocks
+  | 'layers' // layered-composition badge (wireframe) — two stacked planes (one shared space)
   // Mark sub-family (composition wireframe leaves) — a simplified glyph of a unit
   // view's mark type, so which-is-which reads at a glance. Vega-Lite mark synonyms
   // collapse onto these via `markIconName` (CompositionWireframe); unknown → generic.
@@ -177,6 +178,21 @@ const GLYPHS: Record<IconName, ReactNode> = {
       <rect x="9" y="17" width="14" height="5" />
     </>
   ),
+  // Two offset planes — a layer is one space with several marks stacked in z-order.
+  layers: (
+    <>
+      <rect x="6" y="6" width="15" height="15" fill="none" stroke="currentColor" strokeWidth={2} />
+      <rect
+        x="11"
+        y="11"
+        width="15"
+        height="15"
+        fill="none"
+        stroke="currentColor"
+        strokeWidth={2}
+      />
+    </>
+  ),
   // Mark glyphs: simplified renderings of each Vega-Lite mark, drawn on the same
   // 32-grid. Stroke-based where a line reads truer than a fill (line/rule/generic).
   'mark-bar': (