Add Apple graph commands and queries

Once now introduces its first Apple build graph. You can declare Apple targets (libraries, frameworks, applications, and test bundles) in once.toml, inspect the graph with once query targets, once query capabilities, and once query schema, and materialize targets using cache-backed once build, once run, and once test commands. Built-in Apple rule metadata is provided through a Starlark prelude and documented in a new Apple graph guide. This is the first step toward Xcode-backed build actions; it currently runs local materialization actions and writes artifacts under .once/out/<target>/.