Embeddable Once cache SDK for Rust and Apple platforms

Once now ships as an embeddable cache SDK. Rust projects can depend on the once crate directly from GitHub, and Apple projects can use the newly published Once.xcframework.zip release asset through a SwiftPM binary target. The framework exposes a small C ABI and a Swift wrapper (OnceCache) for reading, writing, and checking blobs, plus storing and retrieving action-cache results. Release assets are signed, notarized, and uploaded by CI.