perf: packing algorithm runs too frequently

[McManning]

The packing algo in useCellList will execute ~4x each time we click a NoteAnchor due to various focus changes, container height changes, and cell position changes that all happen in various render cycles. Since there's a sort in there as well, performance isn't going to scale well.

Throttle the packing algo, or figure out some other way to reduce call count through some memoization magicks.

[McManning]

For anything that isn't in the current view, the threads really don't need to be shifted around. They shouldn't be too far off from their targets to make it a huge problem. So maybe some basic windowing logic here to make sure we're only acting on visible threads.