AutoScroller scrolls sibling draggables

Currently, AutoScroller will scroll any scrollable container at the pointer position, as long as one exists.

This behaves strangely in a sortable list where items are themselves scrollable: when you drag an item and hover over another sibling item, the inside of that sibling gets unexpectedly scrolled.

**Proposals**

Only scrolling the source's ancestor containers and droppables seems like a reasonable default behavior to me.

Alternatively, having the AutoScroller accept a `shouldScroll` predicate (signature `(scrollable, source) => boolean`) and letting consumers decide the trigger condition is also a good direction.

WDYT?

I'd be happy to contribute once we have a plan.

Start by locating AutoScroller and reproducing the sortable-list case where a dragged item hovers over a scrollable sibling. Review how the source, ancestors, and droppables are identified, then confirm the intended behavior with maintainers: restrict scrolling to relevant containers or support a shouldScroll predicate, with sibling scrolling no longer occurring.

Written by the indexing model from the issue text.