Add standalone overlay bundle for hosted loader
- Create src/standalone/adapters/ with api.ts and storage.ts
- Create overlay-standalone.ts adapted from overlay.ts
- Create index.ts entry point exposing window.MarginOverlay
- Add vite.standalone.config.ts for IIFE bundle build
- Add build:standalone and build:all scripts to package.json
- Add overlay-api-analysis.md with API requirements analysis
- Update .gitignore to exclude dist-standalone/