Function create_diff_baseline_if_absent 

pub fn create_diff_baseline_if_absent(
    box_dir: &Path,
    rootfs: &Path,
) -> Result<()>

Persist the first pristine baseline for a box without ever replacing it.

Runtime callers invoke this after all host-side rootfs preparation and before the workload starts. Later starts and monitor recovery therefore preserve the original generation’s baseline instead of racing a fast container command.