Round-1 review nits. The constant now only gates the diagnostic `image_heavy`
flag (not routing), so the old name was misleading. Rename + reword its comment
to state the diagnostic-only intent. Also add a classify_pdf symmetry test
(`test_classify_pdf_image_heavy_clean_text_stays_fast`) pinning that a full-page
raster image with a clean text layer routes fast on the classify_pdf path too.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>