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This PR adds a `unstable_reactFragments?: Set<FragmentInstance>` property to DOM nodes that belong to a Fragment with a ref (top level host components). This allows you to access a FragmentInstance from a DOM node. This is flagged behind `enableFragmentRefsInstanceHandles`. The primary use case to unblock is reusing IntersectionObserver instances. A fairly common practice is to cache and reuse IntersectionObservers that share the same config, with a map of node->callbacks to run for each entry in the IO callback. Currently this is not possible with Fragment Ref `observeUsing` because the key in the cache would have to be the `FragmentInstance` and you can't find it without a handle from the node. This works now by accessing `entry.target.fragments`. This also opens up possibilities to use `FragmentInstance` operations in other places, such as events. We can do `event.target.unstable_reactFragments`, then access `fragmentInstance.getClientRects` for example. In a future PR, we can assign an event's `currentTarget` as the Fragment Ref for a more direct handle when the event has been dispatched by the Fragment itself. The first commit here implemented a handle only on observed elements. This is awkward because there isn't a good way to document or expose this temporary property. `element.fragments` is closer to what we would expect from a DOM API if a standard was implemented here. And by assigning it to all top-level nodes of a Fragment, it can be used beyond the cached IntersectionObserver callback. One tradeoff here is adding extra work during the creation of FragmentInstances as well as keeping track of adding/removing nodes. Previously we only track the Fiber on creation but here we add a traversal which could apply to a large set of top-level host children. The `element.unstable_reactFragments` Set can also be randomly ordered.