Apple build graph targets and commands

Once now understands Apple targets in its build graph. You can declare Swift, Objective-C, C, and C++ libraries, frameworks, applications, and test bundles for Apple platforms in once.toml manifests, then query their schemas, build them, run apps on macOS or the iOS simulator, and run tests through the same CLI. Built-in Apple target kinds are implemented as Starlark prelude rules that lower into cacheable actions, with outputs placed under .once/out//.