[DOM] Unobserve fragment IntersectionObserver targets after exit (#37408)

Fragment IntersectionObserver targets used to stay observed after a
child was removed so the exit record (isIntersecting: false) could still
fire, but that left the observer holding detached nodes. We now
unobserve ResizeObserver targets immediately, and delay
IntersectionObserver unobserve until after paint so the exit still
lands, then drop the strong ref.

If the same node is reinserted before that flush, we cancel the pending
unobserve so a later cleanup does not detach a child that’s still
visible.

Closes https://github.com/react/react/pull/37452/
Closes https://github.com/react/react/pull/37302
Fixes https://github.com/react/react/issues/37451