Ship embeddable cache SDK with signed Once.xcframework

Once 0.14.0 ships an embeddable cache SDK, so you can use Once cache access directly from your own Rust or Apple apps. The Rust API lives in the new once crate, and Apple apps can link a signed and notarized Once.xcframework with a small C ABI and Swift bindings. The SDK is focused on cache access; the CLI keeps its existing Microsandbox provider support, while SDK consumers opt into that provider only when needed. Release assets now include the packaged XCFramework alongside the existing binaries.