Apple build graph commands and queries

The first Apple build graph slice is now available. You can declare Apple targets in once.toml using kinds such as apple_library, apple_framework, apple_application, and apple_test_bundle, then inspect them with once query targets, once query capabilities, and once query schema. The commands once build, once run, and once test materialize artifacts locally through a cache-backed action graph, with outputs written under .once/out//. This is the initial implementation of the build graph RFC and focuses on Apple targets first, without invoking Xcode yet.