[AvdLee]

Summary

• Document the iOS 27 tab bar/sidebar presentation on iPhone: .tabViewStyle(.sidebarAdaptable), defaultTabBarPlacement(_:), and isTabViewSidebarAvailable.
• Extend adaptive-layout guidance: size classes for what to show vs. proposed size for how big, onChange(of: horizontalSizeClass) over onChange(of: geometry.size), avoid branching between self-adapting containers, avoid device-sized fixed frames, cap wide text.
• Add a resize-extremes check to the Duo verification section.
• Route the new topic from SKILL.md.

Verification

• The new snippet type-checks against the iOS 27.2 SDK.
• Behavior and availability match Apple's public SwiftUI reference for isTabViewSidebarAvailable and defaultTabBarPlacement(_:).