Implement dashed vertical lines at column boundaries for debugging
Phase 4.4 column detection. Each column boundary uses a different
color from an 8-color palette with distinct dash patterns for left vs
right boundaries.
- Created render_columns() function in inspect/render/columns.rs
- CSS classes: column-boundary column-left/right for toggleability
- Data attributes: column-index, boundary, x0, x1 for UI consumption
- 10 unit tests covering all functionality
Also fixed pre-existing compilation errors in extract.rs and render
test files where SpanJson/BlockJson structs were missing required
fields (color, confidence_source, flags, rendering_mode, lang, spans).
Closes: pdftract-5bu2k
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
