#87: @available and Deprecation in Swift

Swift can express platform availability and deprecations directly in the source.

@available(iOS 16, macOS 13, *)
func useModernAPI() {}

@available(*, deprecated, renamed: "useModernAPI()")
func useOldAPI() {}

This gives the compiler enough information to warn callers and suggest migration paths.

It is one of the easiest ways to keep an API evolving without leaving users guessing.