iOS 26 native UX patterns (SwiftUI)

Adopt the iOS 26 native UX cluster in SwiftUI — the collapsible floating tab bar (via the Tab(...) declaration + TabBarMinimizeBehavior), the bottom accessory "mini-player" slot (tabViewBottomAccessory + TabViewBottomAccessoryPlacement .inline/.expanded), scroll-edge glass (scrollEdgeEffectStyle), immersive background extension (backgroundExtensionEffect), the zoom navigation transition (navigationTransition(.zoom) + matchedTransitionSource), and the @Animatable macro for Shapes. Documents each iOS gate (some iOS 18, most iOS 26) and the legacy-API fallback. Use when wiring the interactive chrome of an iOS 26 app — tab bar, mini-player, hero transitions, immersive headers…