axis: corner shape (round / squircle / bevel) via CSS corner-shape

[mehdibha]

What

A corner-shape axis on top of the radius system: one --corner-shape variable (round default), consumed as corner-shape: var(--corner-shape, round) by components that carry a radius.

Why

• iOS 26 is squircle-everything; a builder that can't express it can't recreate Apple-flavored systems.
• CSS corner-shape (squircle / bevel / scoop / notch) ships in Chromium — CSS.supports('corner-shape','squircle') is already true there.
• Progressive enhancement is free: unsupported browsers render plain round corners, so there's no fallback work.

Prototype

/internal/panel-lab/v2 → Shape section already has the control (Round / Squircle / Bevel) applied live to its corner preview and slider self-demo.

Notes

• Runtime-switchable var, like every axis — no build-time decision.
• Interaction with the radius role layer (proposal: radius role layer (control/item/surface/panel) + pill #579) is orthogonal: shape of the corner vs size of the corner.
• Bevel/scoop are what make the square characters (lyra/sera-style) interesting — a square system with beveled corners is a distinct look currently unreachable.