Implements three key optimizations to eliminate layout thrashing and
improve drag performance from 5-20fps to smooth 60fps:

1. Pre-calculate all item midpoints at drag start
2. Only check items within the viewport (with 300px buffer)
3. Only recalculate the midpoint for the moved item and neighbors

This code was writting a good while ago and performance optimizations
aren't necesarily my strong suite, so I worked with Claude Code to
digest the code and offer some suggestions