docs(pdftract-2b7ff): add verification note for image_coverage_fraction signal

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.

notes/pdftract-2b7ff.md

@@ -0,0 +1,56 @@
+# Verification Note: pdftract-2b7ff
+
+## Bead: image_coverage_fraction signal evaluator
+
+## Status: PASS ✅
+
+The `image_coverage_fraction` signal evaluator was already implemented in the codebase at `/home/coding/pdftract/crates/pdftract-core/src/classify.rs` (lines 359-413).
+
+## Implementation Details
+
+### Signature
+```rust
+pub fn image_coverage_fraction(ctx: &PageContext) -> Option<Vote>
+```
+
+### Algorithm
+1. Compute page area: `page_area_pt2 = ctx.width * ctx.height`
+2. Guard against zero/negative page area (returns `None`)
+3. Sum all `image_xobject_areas` to get total image coverage
+4. Compute coverage fraction: `total_image_area / page_area_pt2`
+5. Clamp to `[0.0, 1.0]` to handle overlapping images defensively
+6. If `coverage_fraction > 0.85`: return `Some(Vote::scanned(0.85))`
+
+### Trade-offs Documented in Code
+The implementation uses `sum` instead of `union` for simplicity, with a clear comment noting:
+- 5 overlapping copies of one image = sum of 5x area but union is 1x area
+- This is acceptable for the 0.85 threshold (conservative signal)
+- Revisit with Klee's algorithm (~O(N log N)) if accuracy demands
+
+## Acceptance Criteria Verification
+
+| AC | Status | Notes |
+|---|--------|-------|
+| Page with one image covering 90% area → Some(Vote { 0.85, Scanned }) | ✅ PASS | Lines 2698-2713 test this exact case |
+| Page with multiple small images totaling 50% → None | ✅ PASS | Lines 2715-2730 test this exact case |
+| Page with no images → None | ✅ PASS | Lines 2732-2743 test this exact case |
+| Coverage clamped to 1.0 on overlapping images | ✅ PASS | Lines 2745-2766 test 5x overlapping images |
+
+## Additional Tests Verified
+
+| Test Case | Status |
+|-----------|--------|
+| Exactly 85% threshold (just above) | ✅ PASS (lines 2769-2783) |
+| Just below 85% threshold | ✅ PASS (lines 2785-2797) |
+| Zero page area | ✅ PASS (lines 2799-2810) |
+| Negative page area | ✅ PASS (lines 2812-2823) |
+| Multiple images totaling 90% | ✅ PASS (lines 2839-2856) |
+
+## Conclusion
+
+The implementation is complete, correct, and thoroughly tested. All acceptance criteria pass. The bead is ready to close.
+
+## Files Reviewed
+
+- `/home/coding/pdftract/crates/pdftract-core/src/classify.rs` - Main implementation (lines 359-413)
+- `/home/coding/pdftract/crates/pdftract-core/src/classify.rs` - Tests (lines 2696-2889)