feat(react)!: use Oxc for JSX and add React Compiler support

[hanford]

Changes

• Upgrades @astrojs/react to @vitejs/plugin-react v6, using Oxc for JSX transforms and Fast Refresh.
• Adds opt-in experimental React Compiler support with react({ compiler: true }). Requires oxc-transform-react; React 17 and 18 also require react-compiler-runtime.
• Removes the babel integration option. Custom Babel transforms can be configured through @rolldown/plugin-babel in vite.plugins. Includes a major changeset and migration guidance.

Closes #17952. Related to #14955.

Testing

• Adds integration coverage for compiler output, server rendering, and custom Babel transforms before TypeScript is stripped, with the compiler enabled and disabled.
• Adds E2E coverage for hydration and state preservation through Fast Refresh.
• React integration tests pass: 24 passed, 3 existing skips. Build/typecheck, formatting, lint, and frozen-lockfile installation also pass.

Docs

• Updates the integration README and changeset with compiler setup and Babel migration instructions.
• Companion docs PR: docs: document Oxc React Compiler and Babel migration docs#14522.

[changeset-bot]

🦋 Changeset detected

Latest commit: 0b7f8c4

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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@astrojs/react	Major

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[matthewp]

I don't think this is correct, the oxc still runs and strips types before it reaches babel. I don't think this preserves babel compatibility.

I don't think being backwards compatible is important here, we can release this as a major. We just want to document for the user who still wants to use custom babel transformations how they can do so.

[hanford]

@matthewp Agreed, you're right: preserving JSX doesn't preserve Babel compatibility when Oxc has already stripped the types.

I've updated the PR to make this a major release, use @vitejs/plugin-react v6, and remove the babel integration option and compatibility layer. The README, changeset, and docs PR now include migration examples for custom Babel transforms using @rolldown/plugin-babel under vite.plugins.

I also added a TSX regression test that verifies Babel sees TypeScript syntax before Oxc strips it, with the compiler both on and off. The PR is rebased on main with the lockfile conflict resolved, and the React integration and Chrome tests pass.

[matthewp]

Ok much better, will talk about docs with the @ArmandPhilippot about the docs and see when we can get this in.

[hanford]

@matthewp Do you have a rough sense of when this might land e.g., days or weeks? Selfishly, I’m excited to use it myself 🙂 No rush, just figuring out whether to wait or use patch-package with these changes in the meantime.