Address PR #855 round 3 (non-blocking test completeness): - Add a test that a mostly-digital doc with one full-page image carries the image_heavy flag yet still routes fast (ocr_frac < OCR_PAGE_FRACTION) -- the flag-vs-routing asymmetry operators read in the metrics, now guarded against silent regression. - test_full_page_image_routes_ocr also asserts the scanned flag (no text layer). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>