Incremental UI stack

## What problem does this solve or what need does it fill?

The stacking order in `UiStack` is rebuilt every frame. Its implementation as a single ordered list of entities makes its difficult to track local changes.

## What solution would you like?

Update the stacking order incrementally.

Split the stack into local lists that make it easier to track local changes.

Start by locating the `UiStack` implementation and the code that rebuilds its ordered list each frame. Trace how entities are added and removed, then determine how local lists could track those changes without rebuilding the full stack. Done means the stacking order is updated incrementally and the implementation is covered by the relevant UI tests.

Written by the indexing model from the issue text.