Apple graph commands and queries

Once now models Apple targets as a typed build graph. You can declare targets in once.toml using built-in rules like apple_library, apple_framework, apple_application, and apple_test_bundle. The CLI adds once query targets, once query capabilities <target>, and once query schema <rule> to inspect the graph, plus once build <target>, once run <target>, and once test <target> to materialize targets locally. Build writes artifacts to .once/out/<target>/, run consumes an app bundle, and test produces test results and coverage. These local cache-backed actions do not invoke Xcode yet; Xcode-backed execution is planned.