_DarwinFoundation2 cross-import overlay

_DarwinFoundation2 is an internal Swift overlay that bridges a set of Darwin C library headers into Swift. It surfaces POSIX functions for stdio, string handling, ctype character classification, time, and byte order, making these low-level C APIs callable from Swift across iOS, macOS, tvOS, watchOS, and visionOS. The overlay defines no public Swift types of its own; it adapts the underlying C declarations so they interoperate with Swift conventions.