Adds curved arrows between consecutive blocks in reading order with
numeric labels. Arrows use quadratic bezier curves with control points
at midpoint + 10pt downward. Limits to 50 arrows to prevent visual
clutter.
- Add render_reading_order function returning SVG path and text elements
- Include data-* attributes for tooltip consumption
- Add comprehensive unit tests (10/10 passing)
- Export reading_order module from inspect/render/mod.rs
Acceptance criteria:
- Helper compiles and produces valid SVG output ✅
- Layer is independently toggleable via CSS class ✅
- data-* attrs populated ✅
- Unit tests pass ✅
Closes: pdftract-6559n
