Preserved design resizing

Convert a rectangle or ellipse with Edit as path, then enable Preserve design in its Path properties. Fixed left, right, top, and bottom source regions render as a nine-slice grid: corners retain both dimensions, edges stretch along one axis, and only the center stretches freely. This keeps angled bar ends, borders, and corner treatments unchanged as the layer grows. The initial profile supports solid path fills; complex paints continue to use ordinary scaling and produce a validation warning.