I saw that. Unfortunately, that opens up more issues and it still doesn't build. Switching out @main for @UIApplicationMain leads to "@UIApplicationMain may only be used on 'class' declarations. It also is still reporting that the compiler can't find App or Scene in scope.
To use @UIApplicationMain, I'd need to change TestApp from a struct to a class, but then I'll get the error that 'UIApplicationMain' class must conform to the 'UIApplicationDelegate' protocol.