Shipped an embeddable cache SDK for Rust and Apple platforms

Once ships a new embeddable cache SDK for application authors. Rust projects can import the once crate and read or write cache blobs and action results via the local XDG cache. Apple projects can now use a signed Once.xcframework.zip from each release, exposing a Swift-friendly cache API and a C ABI for Objective-C and C callers. The embedded Microsandbox provider is feature-gated so SDK users only pull it when explicitly enabled, while the CLI keeps its existing provider behavior. Release packaging handles code signing, notarization, and artifact upload.