[React 19] Removal of `ReactDOM.findDOMNode`

## Summary React 19 removes the deprecated `ReactDOM.findDOMNode` utility. While this utility was an escape hatch and refs are the recommended way to access the DOM, in certain cases, `findDOMNode` was the only solution using a React API. Let me explain. I maintain `@makeswift/runtime`, hosted at https://github.com/makeswift/makeswift, which is the SDK for [Makeswift](https://www.makeswift.com/), a visual editor for Next.js and React. Users of Makeswift can register React components with Makeswift so that they're available to drop in the Makeswift builder. We leverage `findDOMNode` so that we can find registered component's DOM nodes when users don't use `forwardRef` (or in React 19, handle the `ref` prop). Here's how we use `findDOMNode`: https://github.com/makeswift/makeswift/blob/58f425cf522c23af4a71b2f07a7625b252c59a5e/packages/runtime/src/runtimes/react/find-dom-node.tsx While we could force users to use `forwardRef`, a very important product philosophy of Makeswift is that we meet developers where they're at. We don't want them to have to make _any_ changes to their components. Their components shouldn't know about Makeswift, and if they weren't forwarding a ref then introducing Makeswift shouldn't make them do so. In the same vein, we also don't want to alter the DOM in any way, so alternatives like using a `div` with `display: contents` are a no-go for us. Ideally, we'd be able to use refs with a `React.Fragment`, but that API doesn't exist yet. This leaves us with the alternative of requiring users to forward refs in their components or reaching for React internals, like React DevTools does to associate a DOM node with rendered components. We'd like to avoid using internals but without `findDOMNode`, that might be what we have to do since we'd rather do that than force users to have to forward refs. Would love to hear thoughts and guidance on what the expectation is for library maintainers to do for this use case that can't be handled with refs. Is the sugg…