SvgExportDef

A <defs> entry a shape needs present in the exported SVG — a gradient, a marker, a filter, a pattern.

Collected across every shape being exported and de-duplicated by key, so ten thousand arrows share one arrowhead marker rather than carrying one each. key is therefore the identity of the *definition*, not of the shape: two shapes returning the same key must mean the same thing by it.

getElement may be async because a definition can need something fetched — an embedded font, an inlined image — and the export waits for all of them before it serializes.