Previously color/shading detection only ran as fallback when no line-based
boxes were found. Now both methods run in parallel with result merging,
so smaller shaded boxes (like "German leihen") get detected even when
larger bordered boxes are already found. Uses median-blur background
analysis that works for both colored and grayscale/B&W scans.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>