Add SVG image support

[rowland]

Summary

• add SVG-as-image support across svg, pdf, and ltml
• route SVG through the existing image APIs and LTML <image> path
• add samples, fixtures, tests, and docs for the supported SVG subset

What changed

• added a new svg package for XML parsing, style parsing, transforms, scene modeling, and path parsing including elliptical arcs
• added explicit SVGDimensions* and PrintSVG* APIs, plus SVG auto-detection in the existing image APIs
• added PDF-side SVG rendering for shapes, paths, clip paths, and basic text
• added LTML integration so <image src="...svg"> works unchanged
• added docs, samples, a golden PDF, and a tmp-SVG sample runner for visual verification
• fixed follow-up issues around class-based internal stylesheet fills and metadata-only elements like <title> / <desc>

Verification

• go build ./...
• go test ./svg ./pdf ./ltml
• go run ./samples test_016_tmp_svgs