chore: prepare verified CRX uploads and release 0.15.4

- Add helper scripts for Chrome Web Store verified CRX uploads using a dedicated RSA upload key protected by GPG.
- Document the verified upload packaging flow and ignore local signing secrets.
- Add npm packaging entry point for signed webstore CRX artifacts.
- Chunk large SDK tab close batches to avoid native-host response timeouts.
- Bump project and extension versions to 0.15.4 with matching tests.

package.json

@@ -9,6 +9,7 @@
     "check:extension": "tsc -p tsconfig.json --noEmit && npm run build:extension && node --check extension/background.js && npm run test:extension",
     "package:extension": "npm run build:extension && python scripts/package_extension.py",
     "package:extension:webstore": "npm run build:extension && python scripts/package_extension.py --webstore",
+    "package:extension:webstore:verified": "scripts/package_verified_crx.sh",
     "package:extension:firefox": "npm run build:extension && python scripts/package_extension.py --firefox"
   },
   "devDependencies": {