vellumwidget: Interactive Web Widgets for Vellum Scenes

Turns a 'vellum' scene (or a 'vellumplot' plot) into a self-contained, client-side interactive HTML widget: hover tooltips and highlighting, click selection, rectangular brush-select, pan/zoom, and a toolbar – driven by the per-element data keys, bounding boxes, and 'scene_model()' metadata that 'vellum' emits. Requires no Shiny and no server round-trip.