Add Apple build graph and query commands

Once now introduces a typed build graph for Apple projects. You can model Apple libraries, frameworks, applications, and test bundles in once.toml, and the new CLI supports once build, once run, and once test for Apple targets. The work also adds query commands so you can inspect targets, capabilities, and schemas, and it includes a Starlark prelude with built-in Apple rule metadata. Everything is aligned with the build graph RFC, so scripts remain a first-class migration path into the typed graph.