Replace AdjustColumn stub with a convergence loop that distributes the
remaining gap between the topmost part and the work area top edge across
all pairs. TryAdjustPair/TryShiftDirection try shifting part2 up (or down
as fallback) and compact left, rejecting moves that widen the pair.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
