Embeddable cache SDK with signed XCFramework release

Once now ships a new embeddable cache SDK that lets you call the cache from code outside the CLI. It includes a Rust crate with a documented Rust API, a C/FFI bridge with Apple module headers, and a Swift wrapper. You can embed the local cache via the Rust API, the C ABI, or a SwiftPM binary target built from the signed Once.xcframework.zip release artifact. Releases are now packaged as a signed, notarized, and codesigned XCFramework. The embedded Microsandbox provider is gated behind a feature flag, so SDK consumers only pull it in when they opt in. The CLI keeps its existing provider support by default.