Support SVG <use> references

[rowland]

Problem

Some SVGs still warn with svg: <use>: unsupported element skipped. Those files rely on symbol/shape reuse via <use>, so skipping the element drops visible content.

What implementation would require

• Parse and retain reusable elements under <defs> by id.
• Resolve <use href="..."> and legacy xlink:href references.
• Clone the referenced node subtree while applying the <use> element's own x, y, transform, and style overrides.
• Make sure inherited style, clip-path, and transforms compose correctly with the referenced content.
• Decide whether unresolved references should warn and skip or hard-fail.
• Add tests for simple shape reuse, nested groups, and style overrides.

Workarounds

• Expand <use> references before handing the SVG to leadtype. Many vector tools can outline or expand symbols during export.
• Save a PDF-targeted SVG variant with duplicated geometry instead of symbol references.

[rowland]

Status audit (2026-08-14): most of the requested <use> behavior landed incidentally in ce03ef6 / #88. Supported today: retaining supported shapes/groups under <defs> by id, resolving both href and xlink:href, composing x/y, transforms, inherited style overrides, clip paths and masks, and warning/skipping on missing references.

Keeping this issue open because <symbol> elements are still unsupported, parsed <use width>/height values are not applied (important for symbol/viewBox reuse), and there is no focused regression test for a referenced nested group. The modern href form and style-override behavior also lack direct assertions. The checked-in SVG corpus only exercises direct shape references and does not contain <symbol>.