One SwiftUI layout, three native idioms — keeping an AI agent from shipping "iPhone UI with a bigger window" on Mac and iPad

Posting this here rather than dev.to because I think the actual interesting part of this story is a Swift problem, not a marketing one: what happens to platform idiom — tab bar vs. sidebar, stacked panes vs. side-by-side, a titled window vs. no window at all — once the thing writing your SwiftUI is an AI agent instead of you. Quick context for anyone who hasn't run into DesignFoundation (MIT, free) or DesignFoundationPro (paid) before: they're a token-based SwiftUI design system and a set of pr...