dillon1000/react

Allow uncached IO to stablize (#25561)

Initial draft. I need to test this more.

If a promise is passed to `use`, but the I/O isn't cached, we should
still be able to unwrap it.

This already worked in Server Components, and during SSR.

For Fiber (in the browser), before this fix the state would get lost
between attempts unless the promise resolved immediately in a microtask,
which requires IO to be cached. This was due to an implementation quirk
of Fiber where the state is reset as soon as the stack unwinds. The
workaround is to suspend the entire Fiber work loop until the promise
resolves.

The Server Components and SSR runtimes don't require a workaround: they
can maintain multiple parallel child tasks and reuse the state
indefinitely across attempts. That's ideally how Fiber should work, too,
but it will require larger refactor.

The downside of our approach in Fiber is that it won't "warm up" the
siblings while you're suspended, but to avoid waterfalls you're supposed
to hoist data fetches higher in the tree regardless. But we have other
ideas for how we can add this back in the future. (Though again, this
doesn't affect Server Components, which already have the ideal
behavior.)