Add React 19.3 adoption codemods and recipe

[mohebifar]

Summary

React 19.3 (release post) has no breaking changes, so these are adoption codemods: they move code from experimental names and pre-19.3 workarounds onto the stable APIs.

New packages under codemods/:

Package	What it does
react-19-3-unprefix-stable-apis	Renames unstable_ViewTransition, unstable_addTransitionType (19.3), unstable_Activity, experimental_useEffectEvent (19.2) to their stable names. Covers named imports with/without aliases, React.unstable_X member access and JSX member tags, CJS require("react") destructuring, re-exports, and all usage sites. Keeps the old local name when the stable name is already taken in the file. Leaves unstable_startGestureTransition alone.
react-19-3-use-browser	Rewrites the useState(false) + useEffect(() => setMounted(true), []) + if (!mounted) return <Fallback /> idiom into use(browser()) from react-dom. The component is split so the old early return becomes a <Suspense fallback> in a wrapper that keeps the original name and export; the inner component calls use(browser()). Handles function declarations, arrow components (including FC<Props> annotations), destructured props, namespace hook access (React.useState → React.use / React.Suspense), and cleans up unused useState / useEffect / useLayoutEffect imports. Bails when the flag is used elsewhere or the fallback references destructured props.
react-19-3-fragment-ref-wrappers	Replaces <div ref={r} style={{ display: "contents" }}> / <span …> wrappers with <Fragment ref={r}>. Only fires when every use of the ref in the file is its declaration, this wrapper's ref prop, or r.current.<FragmentInstance method>. Adds a Fragment import (or uses React.Fragment) as needed, keeping a leading "use client" directive first.
react-19-3-migration-recipe	Chains the three above plus the existing react-19-remove-context-provider.

Also: lockfile importer entries for the new workspace packages, and root README sections.

Test plan

• CI: pnpm run check-types passes for all packages.
• CI: JSSG snapshot tests pass for the three new transforms (10 / 9 / 8 fixtures, including no-change cases for shadowed names, other modules, unsupported ref usage, and extra props), and the recipe metadata tests pass.
• All pre-existing codemod suites still pass.

Note: the session that authored this could not reach the npm registry, so the fixtures were traced by hand and first executed in CI. The only correction needed was a trailing newline in the metrics.json fixtures.

🤖 Generated with Claude Code

https://claude.ai/code/session_01C1zn9AbvKVCERf15x3zJ68