[OnurGumus]

React.toLit renders with ReactDom.render, which React 18 deprecated and React 19 removed. On React 19 the directive raises and renders nothing:

TypeError: react_dom.render is not a function
    at Lit.React.js:46:31
    at RefDirective._updateRefValue (lit/directives/ref.js)

This switches it to createRoot from react-dom/client.

What changes

• src/Lit.React/Lit.React.fs: the directive keeps one root per element. A new root is created only when lit hands the directive a different element, and the old one is unmounted then. The root is also unmounted when the directive disconnects. Fable.React 9.4.0 binds only the old react-dom entry point, so the two members of react-dom/client this needs (createRoot, and a root's render/unmount) are declared in the file.
• package.json: react and react-dom go to ^19, because the sample renders a Feliz component through this directive. The docs toolchain (nacara, gatsby 3) declares react 16/17 as a peer, so npm refuses the install outright. An overrides entry points react and react-dom at the repo's own versions. The lock file changes only for that (under 100 lines, written with npm 10 as CI uses).
• test/react-check/: a manual check page, run with npm run check:react. npm test can't load React: it ships CommonJS and reads process.env at import time, and the test runner serves ES modules to a browser that has neither. The page renders a component through the directive, renders it again with new props, and leaves the result on window.__result.

Requires React 18 or newer. Apps still on React 17 should stay on the current Fable.Lit.React.

How I checked it

• dotnet build Fable.Lit.sln and npm test (37 passed), the same steps as test.yml.
• npm run check:react on React 19.3.1: the page renders "one", then "two" after new props, with no errors. With the old Lit.React.fs the same page throws the TypeError above.
• The sample (sample/App.fs) on React 19.3.1: the React card renders, its useState counts clicks, hiding it runs its effect cleanup, and showing it again mounts a fresh one.

Not covered

• Docs build. npm run docs doesn't work on main today on current Node: on Node 25 nacara passes an option to rmdir that Node removed, and on Node 22 it fails looking for build/docs. With this change it also fails on import "react-dom/server.js", which React 19 doesn't export. Nothing in CI builds the docs, so this PR doesn't break a working pipeline. Whoever revives the docs will still need to replace or patch nacara for React 19.
• React state across lit re-renders. lit hands the directive a new wrapper element on each re-render of the surrounding template, so the React component is mounted afresh and loses its state. That was equally true with ReactDom.render, and fixing it is a bigger change than this.

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