Since Xcode 26.1, actool ignores an .appiconset that shares its name with
an Icon Composer .icon and renders the icons for older OS versions from the
.icon instead. The undocumented --enable-icon-stack-fallback-generation flag
that used to turn this off is now parsed and discarded, so building with
Xcode 27 replaced our flat icons with generated ones on macOS 12-15, where we
prefer our own low resolution art.
Replacing AppIcon.icns is not enough because macOS reads the icon from
Assets.car before Tahoe as well. Instead compile AppIcon.appiconset into its
own catalog and merge its renditions into the built Assets.car with CoreUI,
leaving the icon stack renditions that macOS 26 uses in place.
The rendition format is undocumented, so the merge fails the build if the
catalog does not look the way we expect, and checks that the icons still
decode after writing.
Assisted-by: Claude:claude-opus-5[1m]
