Function move_zone 

fn move_zone(
    filestore: &FileStore,
    old_zone: &str,
    new_zone: &str,
) -> Result<(), String>

Move every file in old_zone to new_zone, preserving names + bytes. Implemented as read-write-delete because FileStore doesn’t expose a native rename; the cost is bounded by the per-zone file count (1-2 in practice — output.state.json + output).