Implements 7.2.3: span-to-cell assignment using centroid containment.
- Add Cell and TableSpan types with bbox, content, row/col indices
- Implement assign_spans_to_cells() with half-open interval [x0, x1)
- Extend edge cell bboxes by 0.5pt to capture spans flush to borders
- Sort cell content by (round(y0/2), x0) with 2-pt y-bucket
- Emit diagnostic when span overlaps adjacent cell by > 40%
- Handle orphan spans (returned separately, not lost)
Adjustment: Changed overlap diagnostic threshold from 50% to 40%
because with half-open intervals, it's mathematically impossible
for a span's centroid to be in one cell while overlapping another
by > 50%.
All 20 unit tests pass including critical 5×3 bordered table test.
Refs: pdftract-2oqh, plan 7.2 line 2591
4cc50f8add