Fix repeated evaluation on scroll

[wtmoose]

Breaking Changes

• Minimum deployment target raised to iOS 18. iOS 17 is no longer supported.
• HeaderContext converted from a struct to an @Observable class to address performance issues related to SwiftData queries. The observable class shields content views from re-performing queries and re-evaluating body on every frame while scrolling. If you stored HeaderContext (or its typealiases MaterialTabsHeaderContext / StickyHeaderContext) by value, you will need to update to reference semantics.
• How to scroll MaterialTabScroll progammatically? #19 MaterialTabsScroll joint scroll position API replaced with iOS 18 ScrollPosition. The old scrollAnchor parameter is replaced by optional scrollPosition and anchor bindings for joint scroll position manipulation between the library and client code.

Improvements

• [Suggestion] Expose TabBarModel and HeaderModel as public. #25 MaterialTabBar now supports an alignment parameter (.leading, .center, .trailing) for controlling horizontal positioning of self-sized tabs when fillAvailableSpace is false.
• [Suggestion] Expose TabBarModel and HeaderModel as public. #25 New MaterialAccessoryTabBar component for adding optional leading and trailing accessory views alongside tab selectors. Accessories scroll horizontally with the tabs.
• [Suggestion] Expose TabBarModel and HeaderModel as public. #25 TabBarModel and HeaderModel are now public, enabling fully custom tab bar implementations via the environment.

[wtmoose]

This is just existing code extracted into a reusable view shared by MaterialTabBar and MaterialAccessoryTabBar (new)