Function split_named_range 

pub fn split_named_range(
    doc: &mut AutoCommit,
    record_id: &str,
    left_end: &HistoryEntryId,
    right_start: &HistoryEntryId,
    left_label: &str,
    right_label: &str,
) -> Option<(String, String)>

Split one named band into two adjacent named bands in the current transaction. The caller supplies both boundary ids because a CRDT history is not inherently linear at concurrent branches; guessing adjacency here would make the operation non-deterministic across peers.