- util/sync-version.js rewrites js-base64@X.Y.Z CDN URLs in
  README.md to the package.json version; wired into the npm
  "version" lifecycle hook so release commits include it
- catch README up to 3.9.2 and fix the stale build description
  (both outputs now compile from base64.ts via rollup)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>