Support `experimental.useExperimentalReact` to opt into React's experimental channel

[github-actions]

Summary

Next.js added a new experimental.useExperimentalReact config option that opts the app into React's experimental release channel without requiring a side-effect feature like experimental.taint to be enabled. The experimental React build emits <link rel="expect"> to hold first paint until the streamed shell is coherent, avoiding flicker from partially-streamed HTML.

Previously, opting into the experimental channel required enabling an unrelated feature (taint, transitionIndicator, or gestureTransition) as a side effect. The new flag is a direct opt-in.

Behavior

• experimental.useExperimentalReact: true selects the react@experimental build (alongside the existing taint/transitionIndicator/gestureTransition triggers)
• It is opt-in only: an explicit false cannot disable the experimental channel when another feature still requires it (the taint APIs only exist in the experimental build); assignDefaults warns on that contradiction
• Feeds the existing needsExperimentalReact aggregation in packages/next/src/lib/needs-experimental-react.ts

vinext implications

• Add the option to vinext's config schema/types so user configs don't error
• Decide whether to wire it up to a React channel switch (vinext currently doesn't ship the experimental React build by default); at minimum, accept the flag without crashing and document the support level
• If we do support it: validate the same contradiction-warning behavior when set to false alongside taint/transitionIndicator/gestureTransition

Upstream

• Commit: vercel/next.js@71b70e0
• PR: Add experimental.useExperimentalReact to opt into React's experimental channel vercel/next.js#94861
• Test: test/e2e/app-dir/rsc-basic/rsc-basic-use-experimental-react.test.ts