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

Once now models Apple projects as a typed build graph, following RFC 0001. Users can declare targets in once.toml with built-in Apple rules such as apple_library, apple_framework, apple_application, and apple_test_bundle. The CLI adds inspectable queries: once query targets, once query capabilities <target>, and once query schema <rule>. It also adds cache-backed materialization commands: once build, once run, and once test for Apple targets, writing artifacts under .once/out/<target>/. The work ships a Starlark prelude for built-in rule metadata and new documentation at docs/guide/apple-graph.md.