Function reflections_contract 

fn reflections_contract(
    lower: MatRef<'_, Complex>,
    upper: MatRef<'_, Complex>,
) -> bool

Certify ||L U||_inf < 1 using |L| |U| 1, in quadratic work.

The entrywise absolute-value bound ignores cancellations, so failure says nothing about invertibility. Such systems retain the dense-product test below.