Add Apple graph commands, queries, and build graph support

Adds a new Apple build graph to the Once CLI and frontend. You can now define Apple targets (libraries, frameworks, applications, and test bundles) in once.toml, inspect them with new graph query commands like once query targets, once query capabilities, and once query schema, and run cache-backed build, run, and test commands that materialize outputs under .once/out//. The Apple model covers headers, SDK frameworks and dylibs, resources, asset catalogs, Info.plist substitutions, entitlements, provisioning profiles, signing, test hosts, test plans, and privacy manifests. Built-in Apple rule metadata lives in a Starlark prelude and is documented in a new guide.