Function compose_split 

pub fn compose_split(
    left: &Frame,
    right: &Frame,
    left_w: u16,
    cols: u16,
    focused_left: bool,
) -> Frame

Stitch two half-width pane frames into one full-width frame: left cells | divider | right cells per body row, per-pane statuses joined, right pane’s highlight ranges shifted past the divider, row-level dim flattened into cells. Pure.