Configurable attributes with select() for Apple targets

Apple rule attributes marked configurable = True can now use a select value in your once.toml manifest. The value resolves at analysis time based on the target configuration, including platform, sdk_variant, architecture values, and the mac_catalyst token. You can write composite keys such as ios:simulator, and the most specific matching branch wins. A default branch is used when nothing else matches. select on non-configurable attributes is rejected by the schema layer.