Function copy_items 

pub fn copy_items(
    dest_root: &Path,
    plan: &MergePlan,
    cancel: &AtomicBool,
    progress: impl FnMut(usize, usize),
) -> (Vec<String>, Vec<(String, String)>)

Copies the plan’s item folders that dest_root lacks into its images/, calling progress(done, total) after each, and stops early when cancel is set. Returns the folder names copied. A folder that fails to copy is skipped (its partial copy removed) and reported in the second list with the reason.