Editor: composition structure wireframe (read-only)

src/app/components/Icon.tsx

@@ -34,6 +34,7 @@ export type IconName =
   | 'export' // export the workspace to a file — Carbon Download (a file comes out)
   | 'info' // about / information — Carbon Information (outline)
   | 'revert' // revert draft to last published — Carbon Reset
+  | 'structure' // composition-structure wireframe disclosure (preview toolbar) — nested view blocks
   // Pane-toggle sub-family (spec §01A): a panel frame with one region filled, so the
   // glyph shows *which* pane it controls by position (left / centre / right).
   | 'pane-library' // toggle the library pane (left)
@@ -150,6 +151,19 @@ const GLYPHS: Record<IconName, ReactNode> = {
       <rect x="21" y="10" width="4" height="12" />
     </>
   ),
+  // Composition structure: a panel frame (matching the pane family) holding nested
+  // view blocks — two side by side over one wide — the wireframe in miniature.
+  structure: (
+    <>
+      <rect x="4" y="6" width="24" height="2" />
+      <rect x="4" y="24" width="24" height="2" />
+      <rect x="4" y="6" width="2" height="20" />
+      <rect x="26" y="6" width="2" height="20" />
+      <rect x="9" y="10" width="6" height="5" />
+      <rect x="17" y="10" width="6" height="5" />
+      <rect x="9" y="17" width="14" height="5" />
+    </>
+  ),
   moon: (
     <path d="M13.5025,5.4136A15.0755,15.0755,0,0,0,25.096,23.6082a11.1134,11.1134,0,0,1-7.9749,3.3893c-.1385,0-.2782.0051-.4178,0A11.0944,11.0944,0,0,1,13.5025,5.4136M14.98,3a1.0024,1.0024,0,0,0-.1746.0156A13.0959,13.0959,0,0,0,16.63,28.9973c.1641.006.3282,0,.4909,0a13.0724,13.0724,0,0,0,10.702-5.5556,1.0094,1.0094,0,0,0-.7833-1.5644A13.08,13.08,0,0,1,15.8892,4.38,1.0149,1.0149,0,0,0,14.98,3Z" />
   ),