Apple graph targets, queries, and local build/run/test commands

Once now ships a typed Apple build graph for iOS, macOS, watchOS, tvOS, and visionOS projects. You can declare Apple targets in once.toml using built-in kinds such as apple_library, apple_framework, apple_application, and apple_test_bundle. Inspect the graph with once query targets, once query capabilities, and once query schema, and materialize artifacts locally with cache-backed once build, once run, and once test commands. Outputs are written under .once/out//, and script targets remain first-class. This is the first graph slice following the Once Build Graph RFC and does not yet invoke Xcode tooling.