Apple build graph targets can be declared in once.toml

Once now models Apple targets in the build graph, with built-in rule schemas for libraries, frameworks, applications, and test bundles. You can declare these targets in once.toml using attributes such as platform, bundle_id, minimum_os, families, resources, asset catalogs, Info.plist, entitlements, provisioning profiles, SDK frameworks, and SDK dylibs. Built-in rule metadata is defined in the Starlark prelude and lowered into typed Rust graph schemas, so the manifest stays the user-facing surface while the internal model is queryable and typed.