feat(mcp): scaffold package and confirm headless bridge viability

- Add packages/mcp/ with package.json, tsconfig, PLAN.md, CROSS_CUTTING.md
- Extend @pascal-app/core exports map with subpaths (./schema, ./store,
  ./material-library, ./spatial-grid, ./wall) so Node consumers can skip
  the graphics-bound systems re-export chain.
- Add scripts/spike.ts that proves useScene + temporal work in Node with
  a requestAnimationFrame polyfill. Spike passes end-to-end.

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

packages/mcp/tsconfig.json

@@ -0,0 +1,14 @@
+{
+  "extends": "@pascal/typescript-config/base.json",
+  "compilerOptions": {
+    "outDir": "dist",
+    "rootDir": "src",
+    "noEmit": false,
+    "composite": true,
+    "incremental": true,
+    "types": ["node"]
+  },
+  "include": ["src"],
+  "exclude": ["node_modules", "dist", "**/*.test.ts", "scripts"],
+  "references": [{ "path": "../core" }]
+}