dillon1000/react

Parallel transitions: Assign different lanes to consecutive transitions (#20672)

* Land enableTransitionEntanglement changes

Leaving the flag though because I plan to reuse it for additional,
similar changes.

* Assign different lanes to consecutive transitions

Currently we always assign the same lane to all transitions. This means
if there are two pending transitions at the same time, neither
transition can finish until both can finish, even if they affect
completely separate parts of the UI.

The new approach is to assign a different lane to each consecutive
transition, by shifting the bit to the right each time. When we reach
the end of the bit range, we cycle back to the first bit. In practice,
this should mean that all transitions get their own dedicated lane,
unless we have more pending transitions than lanes, which should
be rare.

We retain our existing behavior of assigning the same lane to all
transitions within the same event. This is achieved by caching the first
lane assigned to a transition, then re-using that one until the next
React task, by which point the event must have finished. This preserves
the guarantee that all transition updates that result from a single
event will be consistent.