Pages with two side-by-side vocabulary columns separated by a vertical
black line are now split into independent sub-zones before row/column
detection. Each sub-zone gets its own rows, preventing misalignment from
different heading rhythms.
- _detect_vertical_dividers(): finds pipe word_boxes at consistent x
positions spanning >50% of zone height
- _split_zone_at_vertical_dividers(): creates left/right PageZone objects
with layout_hint and vsplit_group metadata
- Column union skips vsplit zones (independent column sets)
- Frontend renders vsplit zones side by side via flex layout
- PageZone gets layout_hint + vsplit_group fields
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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