Embeddable Once SDK and signed Apple XCFramework release

Once now ships an embeddable SDK that other tools can link into their own binaries. The SDK exposes a Rust API, C ABI entry points, and Apple module headers, plus a Swift wrapper for async cache operations. For Apple projects, releases now include a signed and notarized Once.xcframework.zip that can be consumed as a SwiftPM binary target. The embedded Microsandbox provider is feature-gated so SDK consumers only pull it in when explicitly enabled, while the CLI keeps its existing provider behavior. Rust and Apple integration paths are documented in the README and crate README.