Remove the flatten-with-white-background step from the icon generator.
macOS .icns natively supports PNG with alpha, so the flatten was
unnecessarily filling transparent areas outside the squircle mask
with white, causing visible borders in the Dock and Finder.
Split Sharp pipeline into two stages to fix edge pixel blending.
When chained directly, dest-in composite and flatten don't properly
preserve alpha channel data, causing edges to retain the original
icon color instead of blending with the white background.
Fixes#6323
Replace simple SVG rounded rect (circular arcs) with Apple's actual
continuous-corner shape reverse-engineered from UIBezierPath. This
produces the correct squircle with smooth curvature transitions.
