Implements the span layer renderer for the inspector debug viewer.
Renders SVG outline rectangles for each text span, color-coded by
extraction confidence. Red (< 0.5), yellow (0.5-0.8), and green (> 0.8)
indicate low, medium, and high confidence respectively. Gray indicates
direct extraction without OCR.
Each rect includes data-* attributes for tooltip and click consumption:
- data-text: the extracted text content (XML-escaped)
- data-confidence: confidence score or empty string
- data-font: font name (XML-escaped)
- data-size: font size in points
All 10 unit tests pass. The implementation follows the existing SVG
generation pattern in pdftract-core/src/receipts/svg.rs.
Closes: pdftract-p4vzu
25f1081d7d