The builder captures data-label / data-description / data-lightbox on SVG elements into SvgDocument.annotations, each with the element's AABB in viewBox coordinates (accumulated transform applied — rect/ellipse/image/ line/poly/path bounded; groups skipped, text degenerates to its anchor point). Feeds the Flutter caption/lightbox overlay next. Flutter-free, covered by dart test (6 new cases). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>