_DarwinFoundation1 cross-import overlay

_DarwinFoundation1 is a Swift overlay over Darwin C headers, exposing low-level system definitions to Swift on iOS, macOS, tvOS, watchOS, and visionOS. It surfaces math constants, POSIX error codes, locale, and quality-of-service (qos) definitions, letting you work with these underlying C-level facilities from Swift code. As an overlay, it declares no Swift types of its own; it bridges the imported Darwin C declarations into Swift.