Implements the render_anchors helper that draws block-id text labels at the
top-left corner of each block. Shows the Markdown anchor IDs that downstream
output (Phase 6.5 --md-anchors) will produce.
Key details:
- Function: render_anchors(page_index, page_number, blocks) -> Vec<String>
- Anchor ID format: p{page_number}-b{block_index} (e.g., "p1-b0")
- Text positioned at top-left corner (x0+2, y1-4) with small offset
- Data attributes: data-page-index, data-page-number, data-block-index,
data-bbox, data-kind
- CSS class: "anchor-label" for frontend toggleability
- Font: monospace, 10pt, black (#000000)
All 12 unit tests pass, covering empty input, single/multiple blocks,
positioning, bbox format, XML escaping, page variations, and SVG validity.
Closes: pdftract-5edjj
cdf112a300