Support SVG radial gradients that become elliptical under transforms

[rowland]

Context

The SVG parser now understands radialGradient, and the renderer can resolve radial gradients in the circle-preserving cases. It still rejects radial gradients that become elliptical after gradientTransform, object transforms, or similar non-uniform scaling, because the current PDF path does not yet have the right representation for those cases.

Current behavior

• radial gradients parse and simple cases can resolve
• non-uniformly transformed radial gradients warn/fall back instead of rendering as SVG expects

Why this matters

This is the main remaining writer-side gap for broad SVG gradient parity after the linear-gradient work landed.

Scope

• Add the PDF-side representation needed for transformed radial shadings (likely shading/pattern matrix support or an equivalent approach)
• Wire that through SVG radial gradient resolution
• Cover gradientUnits and gradientTransform interactions in the common cases

Acceptance

• SVG radialGradient renders correctly for non-uniform transform cases that produce elliptical output
• common userSpaceOnUse and objectBoundingBox cases are covered
• add renderer tests proving the transformed radial path instead of fallback behavior