Support SVG linear gradients

[rowland]

Problem

SVG files in our current corpus still warn with svg: <linearGradient>: unsupported element skipped, which causes artwork that depends on gradients to render with incorrect solid-color fallback or not at all.

What implementation would require

• Extend the SVG parser to collect <linearGradient> definitions from <defs> and index them by id.
• Support fill="url(#...)" and stroke="url(#...)" references in style parsing.
• Parse gradient stops, offsets, stop-color, and stop-opacity, including simple class/inline style resolution.
• Respect gradient transforms and gradientUnits for the common objectBoundingBox and userSpaceOnUse cases.
• Add a PDF-side rendering strategy. This likely means either:
  • approximating gradients with axial shadings if the writer can expose that cleanly, or
  • rasterizing only the affected painted object as a fallback if native PDF shading support is too invasive for the first pass.
• Add parser tests plus at least one visual/golden test using a real gradient asset.

Workarounds

• Replace gradients with flat fills in exported SVGs when possible.
• Pre-rasterize gradient-heavy SVG assets to PNG before embedding.
• If only a small number of brand assets need gradients, keep an alternate simplified SVG without gradients for PDF generation.