Apple build graph with queries and local build/run/test commands

Once now introduces a typed build graph for Apple targets. You can declare targets in once.toml using built-in Apple rules such as apple_library, apple_framework, apple_application, and apple_test_bundle, and inspect them with once query targets, once query capabilities, and once query schema. Local cache-backed commands once build, once run, and once test materialize artifacts under .once/out// for supported Apple targets. Script targets continue to execute through the existing action cache while this Apple graph layer builds the foundation for future Xcode-backed execution.