Generated argument files are materialized by the action runner

A new structured cmd_args(…) Starlark primitive lets target kinds declare command-line fragments that need generated argument files. The executor materializes the file before running the action and includes the generated content in the action digest so caching stays correct. For users, this means Windows Rust builds no longer fail with malformed feature flags because the prelude can hand off response-file formatting to the runtime instead of writing files manually.