GPU gradients: linear and radial fills rasterise on the device

The GPU rasteriser gains gradients. Linear and radial gradient fills - the sweeps behind buttons, hero sections, and cards - are now drawn on the device, their colour stops living in a GPU buffer and interpolated per pixel in the fragment shader, reproducing the software rasteriser's exact geometry: the linear axis projected from the box's four corners, the radial distance measured per axis so an ellipse stays an ellipse. Proven to within one level per channel against software, the same guarantee the box shapes carry. With rects, rounded rects, borders, and now gradients on the GPU, most of a page's non-text fill pixels are hardware-rasterised. The GPU aspect moves 11 to 14; text, images, and WebGL remain the software path and the next targets. The overall holds at 57.