Shelf: move geometry build into a system, slim renderer

Follows the renderer/system split documented in wiki/architecture/
renderers.md and systems.md: the renderer must not run geometry
generation. Mirrors the door-renderer/door-system pattern.

- New ShelfSystem reads dirtyNodes in useFrame, retrieves the shelf's
  registered Group from sceneRegistry, swaps its children with the
  output of buildShelfGeometry(node), then clears the dirty flag.
  Geometry rebuild is fully imperative — no React work involved.
- ShelfRenderer is now a thin empty <group> that registers with
  sceneRegistry, marks the node dirty on mount, and carries the
  pointer-event handlers + live transform overrides at the root.
- Wired system into shelfDefinition so RegisteredSystems mounts it
  alongside the renderer.

Net effect: dragging shelf parametric sliders no longer re-renders
the renderer per tick — the system rebuilds meshes at frame cadence
based on dirtyNodes, the inspector's per-field subscription only
re-renders the dragged field, and the rest of the React tree stays
quiet.

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

packages/nodes/src/shelf/definition.ts

@@ -46,6 +46,10 @@ export const shelfDefinition: NodeDefinition<typeof ShelfNode> = {
     kind: 'parametric',
     module: () => import('./renderer'),
   },
+  system: {
+    module: () => import('./system'),
+    priority: 5,
+  },
   preview: () => import('./preview'),
   tool: () => import('./tool'),