fix: prevent duplicate __dropping-elem__ from race condition in handleDragOver

When handleDragOver and handleDragLeave fire in quick succession,
a race condition can cause duplicate __dropping-elem__ IDs in the
layout, leading to infinite recursion in resolveCompactionCollision.

The race condition occurs because:
1. handleDragOver calls setLayout([...layoutRef.current, droppingItem])
   and setDroppingDOMNode(<div/>) — both batched by React
2. handleDragLeave fires before React renders, calling
   removeDroppingPlaceholder() which checks layoutRef.current
3. layoutRef.current hasn't updated yet (stale), so hasDroppingItem
   is false — early return sets droppingDOMNode=null without
   filtering the layout
4. After React renders: layout contains __dropping-elem__ but
   droppingDOMNode is null
5. Next handleDragOver enters the if(!droppingDOMNode) branch and
   adds a second __dropping-elem__ to layoutRef.current

Fix: filter out any existing dropping element from layoutRef.current
before adding the new one in handleDragOver.