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

Once now models Apple targets as a typed, queryable build graph. You can list targets and capabilities with once query targets, once query capabilities, and once query schema, and run Apple builds, tests, and executions through new once build, once run, and once test commands. Output is materialized under .once/out/<target>/. This is exposed via a Starlark prelude and documented in the new Apple graph guide.