Function move_named_boundary 

pub fn move_named_boundary(
    doc: &mut AutoCommit,
    left_record: &str,
    right_record: &str,
    new_left_end: &HistoryEntryId,
    new_right_start: &HistoryEntryId,
) -> Option<(String, String)>

Move the boundary between two named bands while preserving both labels. As with split, the caller resolves the two sides from its displayed causal order and sends exact ids.