Parallel transitions: Assign different lanes to consecutive transitions

[acdlite]

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.