Add Apple graph commands and queries

Apple projects can now be modeled in Once’s build graph. You can declare apple_library, apple_framework, apple_application, and apple_test_bundle targets in once.toml, then inspect them with once query targets, once query capabilities, and once query schema. The graph also supports local cache-backed commands: once build, once run, and once test materialize artifacts under .once/out//. This first implementation uses typed Rust graph schemas, a Starlark prelude for built-in rule metadata, and is documented in a new Apple graph guide. It does not yet invoke Xcode, but it provides the inspectable foundation for Apple build, run, and test workflows.