Iris: masks should carry a shape, rounded rectangle first, or take a
container widget as the mask, with corner alpha multiplied rather than
cut. Design: the mask evaluates the same SDF draw_rounded_rect uses,
nested masks chain and multiply like moves, and a rounded Rect's
.masked() makes the container the mask with one radius by construction.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>