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

Once now introduces a typed Apple build graph for Apple projects. You can declare Apple targets in once.toml using built-in kinds such as apple_application, apple_library, apple_framework, and apple_test_bundle, then inspect the graph with once query targets, once query capabilities, and once query schema. The new once build, once run, and once test commands materialize artifacts locally and write outputs under .once/out//, including app bundles, debug symbols, test results, and coverage records. This first iteration is inspectable and cache-backed, and does not yet invoke Xcode. The bundled Apple rule metadata lives in a Starlark prelude and is documented in the Apple Graph guide.