v0.3.0-alpha: Chromium rasterizer (R4), branching, chapters

- Rasterizer: playwright-core + system Chromium renders dom steps to PNG
  (1280x800, JavaScript disabled — snapshots stay inert). Cached permanently
  on the step (dom_html immutable). CHROMIUM_PATH configurable.
- Video render now includes DOM steps as real frames (verified h264 1280x800,
  0 skipped); guides now show rasterized screenshots for DOM steps
- Branching: per-step branch hotspots (dashed, labeled) jumping to any target
  step; lead gates respected on branch jumps
- Chapters: demo-level chapter menu in player bar
- API: PATCH /api/demos/:id (name, chapters), branches on step patch, both in
  public JSON
- MCP surface: 15 tools (+set_step_branches, +set_chapters)
- Docker: chromium + fonts in runtime image, CHROMIUM_PATH preset

apps/api/package.json

@@ -12,11 +12,12 @@
     "@modelcontextprotocol/sdk": "^1.12.0",
     "fastify": "^5.2.0",
     "pg": "^8.13.1",
-    "zod": "^3.24.1"
+    "zod": "^3.24.1",
+    "playwright-core": "^1.49.1"
   },
   "devDependencies": {
     "@types/node": "^22.10.2",
     "@types/pg": "^8.11.10",
     "typescript": "^5.7.2"
   }
-}
+}