Function resolve_workload_exit_code 

pub fn resolve_workload_exit_code(
    box_dir: &Path,
    provider_exit_code: Option<i32>,
) -> Option<i32>

Resolve a workload exit code without treating a clean provider shutdown as proof that the guest workload succeeded.

Once the private terminal channel is staged, an empty or invalid status means the guest never published a result. A nonzero provider status remains useful crash evidence, but a provider zero is not substituted for missing guest state.