Contents.json had a 512x512 entry with idiom=universal+platform=macos, which is not a valid slot in the modern macOS universal icon format — only 1024x1024 is valid, Xcode generates all smaller sizes from it. The invalid entry caused actool to report '2 unassigned children' and fail to compile the icon into the app bundle. Also removed the alpha channel from the 1024 PNG — macOS app icons must be fully opaque (no transparency).