Implement confirm_columns function that partitions page into candidate
columns (regions between consecutive gaps + before-first + after-last),
counts unique lines whose first span's x0 falls within each candidate's
x-range, and promotes candidates with line_count >= 3 to confirmed columns.
Supporting code:
- ColumnGap struct with lo/hi bounds, width(), midpoint()
- detect_column_gaps function for zero-coverage region detection
- HasFirstSpan trait for first span bbox access
- CandidateColumn struct for tracking x_range and line_count
All 49 column tests pass, including all acceptance criteria.
Bead: pdftract-2rkc1
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
