Add node registry primitives (PR 0.1 of node-registry plan)

Introduces the @pascal-app/core/registry surface that future node-bundle
packages (and external plugins) will use to register node kinds with the
host. No runtime behavior changes — registry is empty until subsequent
PRs populate it.

- types.ts: NodeDefinition, Capabilities, Relations, DragAction, Plugin,
  ParametricDescriptor, Affordance, SceneApi, NodeRegistry. Capability
  configs accept an override escape hatch; additive-only after v1.
- registry.ts: nodeRegistry singleton, registerNode, async loadPlugin.
  Validates kind, schemaVersion, apiVersion; rejects duplicate kinds.
- scene-api.ts: createSceneApi factory wrapping the scene store with
  copy-on-write snapshot semantics for pauseHistory/restore/resumeHistory.
- index.ts: barrel re-exporting the public surface.
- core/index.ts + package.json: export * from registry and add the
  ./registry subpath so consumers can import either way.

Tests (27 cases, all bun:test): registry registration / validation /
plugin loading; SceneApi read/write/dirty/history; lazy snapshot capture
with update/upsert/delete reversal via restore and restoreAll.

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

packages/core/package.json

@@ -16,6 +16,11 @@
       "import": "./dist/utils/clone-scene-graph.js",
       "default": "./dist/utils/clone-scene-graph.js"
     },
+    "./registry": {
+      "types": "./dist/registry/index.d.ts",
+      "import": "./dist/registry/index.js",
+      "default": "./dist/registry/index.js"
+    },
     "./schema": {
       "types": "./dist/schema/index.d.ts",
       "import": "./dist/schema/index.js",