Apple build graph with query and build/run/test commands

Once now supports Apple build targets as its first typed build graph. Users can declare Apple libraries, frameworks, applications, and test bundles in once.toml, and then use new commands to inspect and materialize the graph locally. New once query targets|capabilities|schema commands let you inspect the graph, while once build, once run, and once test run local cache-backed actions that write artifacts to .once/out/<target>/. This provides a typed, queryable foundation for Apple projects without requiring Xcode invocation yet. The feature is documented in the new Apple Graph guide.