feat(mcp): add headless scene bridge with RAF polyfill

SceneBridge class wraps @pascal-app/core's Zustand store for Node,
exposing a clean programmatic API for scene load/mutate/export plus
Zundo undo/redo. Requires a requestAnimationFrame polyfill loaded
before any core import to work around the store's RAF-batched dirty
marking.

- 51 tests, 99.68% line coverage on scene-bridge.ts
- All-or-nothing applyPatch with Zod dry-run validation
- Safeguards against prototype-polluting keys in loadJSON
- Resolves children through the flat nodes dict (handles the
  SiteNode.children-as-objects inconsistency)

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

packages/mcp/src/bridge/node-shims.ts

@@ -0,0 +1,33 @@
+/**
+ * Node-compatibility shims for `@pascal-app/core`.
+ *
+ * The core store uses `requestAnimationFrame` inside `updateNodesAction` (to batch
+ * dirty-marking) and inside the temporal undo/redo subscribe callback. Both are
+ * load-reachable — the subscribe callback registers at module import time.
+ *
+ * This file installs a no-op-if-already-defined polyfill that works both in
+ * Node and in the browser. It MUST be imported FIRST from any module that
+ * transitively loads `@pascal-app/core/store`, otherwise the core module will
+ * throw at import time.
+ *
+ * Side-effectful on import: there is no exported API — just import this file.
+ */
+
+type RafCallback = (timestamp: number) => void
+
+type GlobalWithRaf = typeof globalThis & {
+  requestAnimationFrame?: (cb: RafCallback) => number
+  cancelAnimationFrame?: (id: number) => void
+}
+
+const g = globalThis as GlobalWithRaf
+
+if (typeof g.requestAnimationFrame === 'undefined') {
+  g.requestAnimationFrame = (cb: RafCallback): number => {
+    const now = typeof performance !== 'undefined' ? performance.now() : Date.now()
+    return setTimeout(() => cb(now), 0) as unknown as number
+  }
+  g.cancelAnimationFrame = (id: number) => {
+    clearTimeout(id as unknown as ReturnType<typeof setTimeout>)
+  }
+}