fix: handle decrease in itemCount cleanly

[ajhoddinott]

If itemCount decreases, for example due to filtering, there are two issues:

• SizeAndPositionManager lastMeasuredIndex can point to items that no longer exist, causing "index X is outside of range"
• VirtualList tries to render stale items before refresh() has updated

This fix:

• Clamps lastMeasuredIndex in updateConfig()
• Clamps the scroll offset in refresh()

I introduced a derived validItems to filter out stale items. This can be inlined as
{#each items.filter((item) => item.index < itemCount) as item (getKey ? getKey(item.index) : item.index)}
but I felt the line was long and hard to follow.