Ship an embeddable Once cache SDK with signed XCFramework release

Once now ships an embeddable cache SDK that other languages and Apple apps can call into. Rust projects can add the once crate directly, while Apple projects can consume a pre-built Once.xcframework binary from a release and use the Swift wrapper to read and write blobs through the local cache. The SDK exposes a high-level cache API covering blob and action-cache operations, and includes C ABI bindings and module headers so it can be embedded from other languages too. The release pipeline now also signs and notarizes the XCFramework so it is ready for distribution.