Three-part fix for retina tile seam artifacts:
1. iframe isolation from Hugo Tailwind CSS (box-sizing interference)
2. Use tileserver style.json URL (not inline style — different layer configs)
3. fill-antialias: false on choropleth layer
Shortcode is now a thin iframe wrapper. Map logic in static/map-embed.html.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
