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/nodes/src/fence/definition.ts

@@ -94,7 +94,7 @@ export const fenceDefinition: NodeDefinition<typeof FenceNode> = {
   presentation: {
     label: 'Fence',
     description: 'A straight or curved fence segment with configurable posts and infill.',
-    icon: { kind: 'iconify', name: 'lucide:fence' },
+    icon: { kind: 'url', src: '/icons/fence.png' },
     paletteSection: 'structure',
     paletteOrder: 20,
   },