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/core/package.json

@@ -15,6 +15,31 @@
       "types": "./dist/utils/clone-scene-graph.d.ts",
       "import": "./dist/utils/clone-scene-graph.js",
       "default": "./dist/utils/clone-scene-graph.js"
+    },
+    "./schema": {
+      "types": "./dist/schema/index.d.ts",
+      "import": "./dist/schema/index.js",
+      "default": "./dist/schema/index.js"
+    },
+    "./store": {
+      "types": "./dist/store/use-scene.d.ts",
+      "import": "./dist/store/use-scene.js",
+      "default": "./dist/store/use-scene.js"
+    },
+    "./material-library": {
+      "types": "./dist/material-library.d.ts",
+      "import": "./dist/material-library.js",
+      "default": "./dist/material-library.js"
+    },
+    "./spatial-grid": {
+      "types": "./dist/hooks/spatial-grid/spatial-grid-manager.d.ts",
+      "import": "./dist/hooks/spatial-grid/spatial-grid-manager.js",
+      "default": "./dist/hooks/spatial-grid/spatial-grid-manager.js"
+    },
+    "./wall": {
+      "types": "./dist/systems/wall/wall-footprint.d.ts",
+      "import": "./dist/systems/wall/wall-footprint.js",
+      "default": "./dist/systems/wall/wall-footprint.js"
     }
   },
   "files": [