Pin matching React Canary versions in Sandpack demos

[aurorascharff]

Summary

• pin react and react-dom to the same exact Canary build in all 34 Canary Sandpack manifests
• cover the browser, use, Suspense, ViewTransition, and Fragment references, plus the View Transitions Labs post
• prevent Sandpack caches from resolving identical floating canary tags to incompatible React builds

Why

Sandpack can cache the two floating canary dependencies independently. One browser demo resolved react to 19.3.0-canary-eb8feb71-20260814 and react-dom to 19.3.0-canary-eafeac09-20260819, which fails because the packages require exact version equality.

This pins both packages to the August 14 build, which is available through react.dev's custom Sandpack bundler and includes the browser API used by the new examples.

Verification

• confirmed no floating react or react-dom Canary dependencies remain in source content
• ran the browser docs example against react.dev's custom Sandpack bundler: server rendering showed the Suspense fallback, then hydration displayed the browser-only editor
• Prettier check on all six changed Markdown files
• heading ID check on all six changed Markdown files
• git diff --check

[github-actions]

Size changes

Details

📦 Next.js Bundle Analysis for react-dev

This analysis was generated by the Next.js Bundle Analysis action. 🤖

This PR introduced no changes to the JavaScript bundle! 🙌