Add Apple build graph queries and local build, run, test commands

Once now supports a typed Apple build graph in early form. You can declare Apple targets in once.toml using built-in kinds such as apple_library, apple_framework, apple_application, and apple_test_bundle, and inspect them with new CLI queries: once query targets, once query capabilities <target>, and once query schema <kind>. The graph model also adds local commands: once build, once run, and once test materialize cache-backed artifacts under .once/out/<target>/. The implementation keeps scripts as a first-class migration path and is designed for agent introspection, with a typed Rust schema and a Starlark prelude that lowers Apple rule metadata into graph data. Note: the current commands do not invoke Xcode yet, they exercise the local graph and action contract.