Remainder items were being filled into the full remnant box without
compaction. Added ShrinkFill helper that fills then shrinks the box
horizontally and vertically while maintaining the same part count.
This matches the strip item's shrink behavior and produces tighter
layouts that leave more usable space for subsequent items.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>