IconRef: add url kind, point registered kinds at palette assets

User pointed at the palette's PNG icons in the bottom toolbar — they
expected the inspector title to use the same artwork, not the iconify
lucide glyphs. Different visual style.

- IconRef gains `{ kind: 'url'; src: string }`. Plain `<img>` render
  in ParametricInspector (no next/image — the inspector is
  `'use client'`).
- All currently-registered kinds switched to URL refs matching their
  palette `iconSrc`:
    fence    → /icons/fence.png
    slab     → /icons/floor.png
    ceiling  → /icons/ceiling.png
    wall     → /icons/wall.png
    spawn    → /icons/site.png
    shelf    → /icons/column.png (placeholder, same as palette)
- Kind-owned panels (slab/ceiling) already pass URL strings to their
  own PanelWrapper; unchanged.

Door/window/item will get URL refs when they register at Stage A.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/editor/src/components/ui/panels/parametric-inspector.tsx

@@ -137,6 +137,12 @@ export function ParametricInspector() {
 
 function renderIcon(ref: IconRef | undefined): React.ReactNode | undefined {
   if (!ref) return undefined
+  if (ref.kind === 'url') {
+    // Plain <img> here so the inspector doesn't pull in next/image's
+    // server-only requirements (the file is `'use client'`). Same
+    // 16x16 box the legacy panels use.
+    return <img alt="" className="h-4 w-4 shrink-0 object-contain" src={ref.src} />
+  }
   if (ref.kind === 'iconify') {
     return <Icon height={16} icon={ref.name} width={16} />
   }