Phase 2 spike: spawn migration (flagged) + new shelf node

The first time registry-driven nodes actually run in the editor.

Spawn migration (under NEXT_PUBLIC_USE_REGISTRY_FOR_SPAWN flag):
- New packages/nodes/src/spawn/ folder with renderer, tool, schema
  (re-exported from core), parametrics, definition, index.
- Spawn definition appended to builtinPlugin.nodes only when the flag
  is set. With the flag off, the Phase 0 shims fall through and the
  legacy SpawnRenderer / SpawnTool keep ownership.
- New no-props SpawnTool reads activeLevelId from useViewer directly,
  matches legacy placement behavior (half-meter snap, singleton-per-
  level, replace-on-reclick).
- Structural parity test (9 cases) validates definition shape +
  schema identity. Pixel-diff defers to Phase 4 alongside more nodes.

New shelf node (no legacy — registered unconditionally):
- ShelfNode schema in core/schema/nodes/shelf.ts (hand-maintained
  AnyNode union for now; Phase 6 derives the union from the registry
  and moves the schema fully into nodes/shelf/).
- packages/nodes/src/shelf/ folder: pure geometry builder
  (buildShelfGeometry returns a Three.js Group of top board +
  brackets), R3F renderer that mounts the built group, no-props
  placement tool, parametrics descriptor (width/depth/thickness/
  height/bracketStyle/color), definition with surfaces.top stackable
  surface for future stacking, and presentation metadata for the
  palette.
- 13 unit tests across schema bounds and geometry behavior.
- Palette wiring: 'shelf' added to StructureTool union + an entry in
  the structure-tools array (placeholder icon, replaced in Phase 4
  when palette is registry-driven).

Framework changes:
- @pascal-app/viewer now exports useNodeEvents from its public barrel
  so node bundles in @pascal-app/nodes can subscribe to node-specific
  pointer events. (Used by spawn renderer; shelf renderer skips it
  for now since useNodeEvents has a hardcoded kind list — Phase 4
  generalizes it via the registry.)
- @pascal-app/nodes gains @pascal-app/viewer as a peer + dev dep so
  node bundles can import from it.

630 tests pass across 76 files (22 new this phase). Editor app
continues to ship green with both legacy spawn and the new shelf
node co-existing through the Phase 0 dispatch shims.

To validate end-to-end in dev:
- bun dev:community → open editor → click 'Shelf' in structure
  toolbar → click to place. Confirms full registry path
  (NodeRenderer dispatch + ToolManager dispatch + sceneRegistry
  byType Proxy).
- Set NEXT_PUBLIC_USE_REGISTRY_FOR_SPAWN=1, restart dev, place spawn
  → visually identical to legacy. Confirms parity.

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

packages/nodes/src/index.ts

@@ -1,11 +1,44 @@
-import type { Plugin } from '@pascal-app/core'
+import type { AnyNodeDefinition, Plugin } from '@pascal-app/core'
+import { shelfDefinition } from './shelf'
+import { spawnDefinition } from './spawn'
+
+/**
+ * Feature flag for the Phase 2 spike. When `NEXT_PUBLIC_USE_REGISTRY_FOR_SPAWN`
+ * is truthy, spawn registers through the registry path; otherwise the legacy
+ * `SpawnRenderer` and `SpawnTool` in viewer/editor packages own the kind.
+ *
+ * Removed in the PR that signs off parity (legacy spawn files deleted in the
+ * same commit). All other built-in node migrations follow the same pattern.
+ */
+function readEnvFlag(name: string): boolean {
+  const env = (globalThis as { process?: { env?: Record<string, string | undefined> } }).process
+    ?.env
+  const flag = env?.[name]
+  return flag === '1' || flag === 'true'
+}
+
+function isSpawnRegistryEnabled(): boolean {
+  return readEnvFlag('NEXT_PUBLIC_USE_REGISTRY_FOR_SPAWN')
+}
+
+function getBuiltinNodes(): AnyNodeDefinition[] {
+  const nodes: AnyNodeDefinition[] = [
+    // Shelf is a new kind — no legacy code to flag against. It ships
+    // unconditionally so users can place it from the tool palette.
+    shelfDefinition as unknown as AnyNodeDefinition,
+  ]
+  if (isSpawnRegistryEnabled()) {
+    nodes.push(spawnDefinition as unknown as AnyNodeDefinition)
+  }
+  return nodes
+}
 
 /**
  * Built-in plugin bundling every node kind shipped with the Pascal editor.
  *
  * Apps load this once at bootstrap (`loadPlugin(builtinPlugin)`) before
  * mounting the viewer. New built-in nodes are added by creating a folder
- * here under `src/<kind>/` and appending its `NodeDefinition` to `nodes`.
+ * here under `src/<kind>/` and appending its `NodeDefinition` to `getBuiltinNodes`.
  *
  * External plugins follow the exact same shape — same `Plugin` type, same
  * `loadPlugin` call path. This is intentional: the API is stress-tested
@@ -14,5 +47,8 @@ import type { Plugin } from '@pascal-app/core'
 export const builtinPlugin: Plugin = {
   id: 'pascal:core',
   apiVersion: 1,
-  nodes: [],
+  nodes: getBuiltinNodes(),
 }
+
+export { shelfDefinition } from './shelf'
+export { spawnDefinition } from './spawn'