dillon1000/react

useMutableSource: "Entangle" instead of expiring (#18889)

* useMutableSource: "Entangle" instead of expiring

A lane is said to be entangled with another when it's not allowed to
render in a batch that does not also include the other lane.

This commit implements entanglement for `useMutableSource`. If a source
is mutated in between when it's read in the render phase, but before
it's subscribed to in the commit phase, we must account for whether the
same source has pending mutations elsewhere. The old subscriptions must
not be allowed to re-render without also including the new subscription
(and vice versa), to prevent tearing.

In the old reconciler, we did this by synchronously flushing all the
pending subscription updates. This works, but isn't ideal. The new
reconciler can entangle the updates without de-opting to sync.

In the future, we plan to use this same mechanism for other features,
like skipping over intermediate useTransition states.

* Use clz instead of ctrz to pick an arbitrary lane

Should be slightly faster since most engines have built-in support.