Add image_coverage_fraction signal evaluator that computes the union
image coverage fraction from individual image XObject areas.
- Computes total image coverage as sum of image_xobject_areas
- Divides by page area (width * height) to get coverage fraction
- Clamps to [0.0, 1.0] to handle overlapping images (defensive)
- Returns Some(Vote::scanned(0.85)) if fraction > 0.85
Implementation uses sum for simplicity (overestimates coverage when
images overlap), which is acceptable for the 0.85 threshold as it's
a conservative signal. Can be revisited with Klee's algorithm for
greater accuracy if needed.
Acceptance criteria PASS:
✓ Page with one image covering 90% area → Some(Vote { 0.85, Scanned })
✓ Page with multiple small images totaling 50% → None (below threshold)
✓ Page with no images → None
✓ Coverage clamped to 1.0 on overlapping images
Also includes pre-existing infrastructure:
- tr3_op_count field in PageContext
- image_xobject_areas field in PageContext
- all_tr3_with_full_page_image function
- CharDensityRatioSignal evaluator
These were necessary dependencies for the new evaluator to function.
Refs: Plan section Phase 5.1.2, coordinator pdftract-22p
