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

Introduces a new build graph layer for Apple targets. You can now declare Apple targets in once.toml for libraries, frameworks, applications, and test bundles. Once supports once build, once run, and once test commands that materialize artifacts under .once/out/<target>/. It also adds once query targets, once query capabilities, and once query schema so you can inspect the graph before running anything. The Apple model is inspired by Bazel rules_apple and Buck2 Apple concepts, but it is not a drop-in replacement for either. For now, the implementation does not invoke Xcode yet.