Embeddable cache SDK with Rust and Apple support

Once can now be embedded in other Rust and Apple applications. Add the once crate from the repository to get a Rust cache client with local blob and action cache operations, or link the Once.xcframework from the release to call a small C ABI from Swift and Objective-C. The SDK intentionally exposes only cache access, and the embedded Microsandbox provider is gated behind a feature flag so SDK consumers only pull it in when they enable it. Release assets are signed, notarized, and published from CI.