Add data-span-index attribute to span rectangles for click navigation
between SVG canvas and JSON-tree panel. Updated render_spans() to use
enumerate() for tracking indices. Added unit tests for index assignment.
Created demo HTML file demonstrating the full click navigation feature:
- Click span rect -> scroll JSON tree to matching entry
- Highlight target node with yellow background for 2 seconds
- Auto-open ancestor <details> elements
- Smooth scrollIntoView with center alignment
Acceptance criteria:
- PASS: data-span-index attribute added to all spans
- PASS: Click handler scrolls tree to matching node
- PASS: .highlighted class applied for 2 seconds
- PASS: Ancestor details auto-opened before scroll
- PASS: 9 unit tests pass including new span_index test
Closes: pdftract-saddv
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
