Structured argument files for declared actions

Rule authors can now pass a structured cmd_args fragment with use_arg_file to declared actions, so generated argument files are materialized by the executor rather than hand-written by each target kind. The feature is included in the action digest so cache keys stay correct. This also fixes the Windows rustc response file issue by keeping arguments like feature=“default” in the literal format rustc expects, and adds coverage for the new command-line contract.