Function run_obj_plan 

pub async fn run_obj_plan(
    plan: &ObjBuildPlan,
    rustc_path: &Path,
    cwd: &Path,
) -> Result<PathBuf>

Spawn rustc with plan.args from cwd. On success, returns the path of the emitted object (= plan.expected_object).

rustc with --emit=obj <path> writes exactly one .o; we don’t need to scan the directory after the fact.