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/ceiling/definition.ts

@@ -86,7 +86,7 @@ export const ceilingDefinition: NodeDefinition<typeof CeilingNode> = {
   presentation: {
     label: 'Ceiling',
     description: 'A polygon-bounded ceiling surface that hosts ceiling-mounted items.',
-    icon: { kind: 'iconify', name: 'lucide:square-dashed' },
+    icon: { kind: 'url', src: '/icons/ceiling.png' },
     paletteSection: 'structure',
     paletteOrder: 40,
   },

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,
   },

packages/nodes/src/shelf/definition.ts

@@ -64,7 +64,7 @@ export const shelfDefinition: NodeDefinition<typeof ShelfNode> = {
   presentation: {
     label: 'Shelf',
     description: 'A horizontal surface for stacking other items.',
-    icon: { kind: 'iconify', name: 'lucide:layers' },
+    icon: { kind: 'url', src: '/icons/column.png' },
     paletteSection: 'structure',
     paletteOrder: 50,
   },

packages/nodes/src/slab/definition.ts

@@ -83,7 +83,7 @@ export const slabDefinition: NodeDefinition<typeof SlabNode> = {
   presentation: {
     label: 'Slab',
     description: 'A polygon-bounded floor surface that hosts items on top.',
-    icon: { kind: 'iconify', name: 'lucide:square' },
+    icon: { kind: 'url', src: '/icons/floor.png' },
     paletteSection: 'structure',
     paletteOrder: 30,
   },

packages/nodes/src/spawn/definition.ts

@@ -52,7 +52,7 @@ export const spawnDefinition: NodeDefinition<typeof SpawnNode> = {
   presentation: {
     label: 'Spawn Point',
     description: 'Player or camera origin within a level. One per level.',
-    icon: { kind: 'iconify', name: 'lucide:flag' },
+    icon: { kind: 'url', src: '/icons/site.png' },
     paletteSection: 'structure',
     paletteOrder: 90, // bottom of structure list — matches legacy palette order
   },

packages/nodes/src/wall/definition.ts

@@ -82,7 +82,7 @@ export const wallDefinition: NodeDefinition<typeof WallNode> = {
   presentation: {
     label: 'Wall',
     description: 'A straight or curved wall segment. Hosts doors, windows, and wall-mounted items.',
-    icon: { kind: 'iconify', name: 'lucide:wall' },
+    icon: { kind: 'url', src: '/icons/wall.png' },
     paletteSection: 'structure',
     paletteOrder: 10,
   },