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

package-lock.json

@@ -19,6 +19,7 @@
         "@modelcontextprotocol/sdk": "^1.12.0",
         "fastify": "^5.2.0",
         "pg": "^8.13.1",
+        "playwright-core": "^1.49.1",
         "zod": "^3.24.1"
       },
       "devDependencies": {
@@ -3197,6 +3198,18 @@
         "node": ">=16.20.0"
       }
     },
+    "node_modules/playwright-core": {
+      "version": "1.61.1",
+      "resolved": "https://registry.npmjs.org/playwright-core/-/playwright-core-1.61.1.tgz",
+      "integrity": "sha512-h7Qlt6m4REp25qvIdvbDtVmD4LqVXfpRxhORv9L0jzETM05p4fuPJ3dKyuSXQxDSbXnmS79HAgi9589lGSpLkg==",
+      "license": "Apache-2.0",
+      "bin": {
+        "playwright-core": "cli.js"
+      },
+      "engines": {
+        "node": ">=18"
+      }
+    },
     "node_modules/postcss": {
       "version": "8.5.19",
       "resolved": "https://registry.npmjs.org/postcss/-/postcss-8.5.19.tgz",