Function map_between 

pub fn map_between(
    index: &HunkIndex,
    from_pane: usize,
    to_pane: usize,
    row: u32,
) -> u32

D.6.b: translate a row from from_pane to to_pane where each pane is a slot index into Hunk::ranges (0/1 in two-way; 0/1/2 in three-way). Pure function of the published HunkIndex + indices; exposed for direct unit testing without round-tripping through RowMapper and as the body all directional aliases call into.