resolveDrawingSvg lowers a DrawingCardDoc to an SVG string: primitive docs
use inline svg or read svgPath (via an injected reader); template docs use
a registered DrawingRegistry handler, so the d2/icon/compare/image children
plug in. Honest DrawErr on no source / unknown template / unreadable path /
empty output. Flutter-free, covered by dart test (7 cases). Not yet wired.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>