Function copy_items 

pub fn copy_items(
    plan: &AddPlan,
    cancel: &AtomicBool,
    progress: impl FnMut(usize, usize),
) -> Copied

Copies the plan’s item folders into the other library’s images/, rewriting each copy’s metadata.json as Eagle does before it is renamed into place. Calls progress(done, total) after each item and stops early when cancel is set (what was copied stays). Needs no library lock.