axis: per-corner radius (asymmetric shapes)

[mehdibha]

What

Allow a design system to round corners asymmetrically — e.g. sharp top-left everywhere, or leaf-shaped cards (opposite corners rounded).

Why

Some brand systems use corner asymmetry as identity. Currently unreachable: our radius vars set all four corners uniformly.

Notes

• Deferred from the radius-ladder work (PRs feat(www): resolve styles.css surface vars on publish #573–feat(www): derive radius ladder from a base length token #575) — parked, no shadcn style uses it, so no parity pressure.
• If built, it composes under the role layer (proposal: radius role layer (control/item/surface/panel) + pill #579): a role resolves to up to four values instead of one.
• Distinct from mechanical per-corner flattening (button groups, attached inputs), which components already handle and which must not be affected.