Implement the blocks layer renderer for the inspector debug viewer.
This renders translucent SVG rectangles for each structural block,
color-coded by block kind per plan §7.9.
Color encoding:
- heading: blue (#3b82f6)
- paragraph: gray (#9ca3af)
- table: teal (#14b8a6)
- list: purple (#a855f7)
- code: orange (#f97316)
- header/footer: light gray (#d1d5db)
- figure: brown (#a52a2a)
- caption: pink (#ec4899)
Each rect includes data-* attributes for tooltip consumption:
- data-kind, data-text, data-level, data-table-index, data-block-index
Also fix pre-existing missing `column` field in SpanJson test fixtures
across spans.rs and confidence_heatmap.rs.
Closes: pdftract-5iouh
