Function format_derived_delta_cell 

pub fn format_derived_delta_cell(
    d: f64,
    ladder: ScaleLadder,
    is_ratio: bool,
) -> String

Format the signed delta cell for a derived row. Mirrors format_derived_value_cell but always carries an explicit +/- sign so the operator can read directionality at a glance. Ratios render with three decimals (+0.100 is +10pp); other ladders route through auto_scale and pick up the scaled unit suffix.