macOS 26 clips every app icon to its own squircle and shows legacy .icns art
inside a glass plate. Our border is a stroke on the tile's rounded rect, so on
26 the ring sits inside a grey rim and thins toward the corners where Apple's
curve and ours diverge.
Add an Icon Composer package (apps/desktop/assets/icon.icon) of canvas-filling
layers the system masks itself: a border band whose outer edge is the canvas
(the mask supplies the outline) and whose inner edge is Apple's mask offset
inward by the brand border thickness, so the band is the same width everywhere
along the visible outline; and the girl (+ commit badge) as a second layer.
Build flavours recolour the fill in icon.json only.
Apple's mask is a circular arc flanked by two cubic easing segments (the
Figma corner-smoothing construction). Fitted to macOS 26.6.2's own render of
a system icon on the 824-on-1024 grid: radius 214px, smoothing 0.645, rms
0.32px / worst 0.81px; a superellipse cannot get under 0.54px rms.
The .icns targets are unchanged: Apple's own 26-era .icns still sit on the
824 grid, and macOS <= 15 keeps showing that artwork.
The new contract test probes the border layer 640 points along the mask:
opaque at thickness-1.5px, transparent at thickness+1.5px. Rendering the
layers with the old r=185.4 corner violates it at 1448 probe points.
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