feat(document-renderer): figure caption formatting with snapshot dates

[transitrix]

Summary

Implement figure caption formatting with snapshot build dates, enabling diagrams embedded in PDFs to be traced back to their source snapshots and build time.

What's included

• formatFigureCaptions() function to process HTML figure elements
• Figures with data-snapshot-id attributes now display captions with formatted snapshot dates
• CSS rules for figure styling in paged-media context
• Comprehensive test suite for caption formatting

How it works

Figures reference snapshots via data-snapshot-id:

<figure data-snapshot-id="snap-001">
  <img src="diagram.svg" />
  <figcaption>System Architecture</figcaption>
</figure>

When metadata includes snapshots:

const metadata = {
  snapshots: {
    'snap-001': { generated_at: '2026-09-02T14:45:00Z' }
  }
};

const html = wrapHtmlForPrintRendering(content, metadata);
// Caption now reads: "System Architecture (Sep 2, 2026)"

Acceptance criteria

• Figures with snapshots display formatted dates in captions ✓
• Missing snapshots don't corrupt output ✓
• Invalid dates handled gracefully ✓
• SVG and raster figures supported ✓
• All tests passing ✓