The image_coverage_fraction signal evaluator was already implemented
in crates/pdftract-core/src/classify.rs. All acceptance criteria verified:
- 90% single image → Scanned with strength 0.85
- 50% multiple images → None (below threshold)
- No images → None
- Overlapping images clamped to 1.0
Implementation uses sum (not union) with documented trade-off,
revisit with Klee's algorithm if accuracy demands.
