Function assemble_reduced_kkt 

pub fn assemble_reduced_kkt(
    a: &CsMat<f64>,
    b: &[f64],
    l: &CsMat<f64>,
    q_bus: &[f64],
    theta_f_inv: &[f64],
    theta_g_inv: &[f64],
    r: usize,
) -> Result<CsMat<f64>>

Assemble the full reduced augmented KKT block (eq. “reduced”): a symmetric indefinite saddle matrix in (Δp_g, Δθ, Δf, Δν, Δη, Δρ) of size 3n + 2m + 1. Useful as a single operator for a direct factorization.