Add Apple build graph queries and cache-backed commands

Once now models Apple targets as a typed, queryable build graph. You can declare targets in once.toml using built-in rules such as apple_library, apple_framework, apple_application, and apple_test_bundle, inspect them with once query targets, once query capabilities, and once query schema, and run cache-backed materialization commands with once build, once run, and once test. These commands write artifacts under .once/out//, including app bundles, dSYMs, test results, and coverage records. This release only includes the graph model and local materialization actions; Xcode-backed compilation, linking, signing, and launching will arrive in later iterations.