feat(pdftract-56vwd): implement build_x0_histogram for column detection
- Add build_x0_histogram() function for 1pt-resolution x0 histogram
- Add HasBBox trait for generic bbox access
- Implement for [f32; 4] and [f64; 4] types
- Clamp out-of-bounds x0 values with diagnostics
- Add 7 tests covering single/multiple spans, clamping, rounding, A4 pages
Acceptance criteria PASS:
- Single span at x0=100: hist[100] == 1
- Multiple spans: hist[100]==2, hist[200]==2, hist[300]==1
- Negative x0 clamped to hist[0] with diagnostic
- Empty spans returns zero Vec
Closes: pdftract-56vwd
8bc63ac8b3