Function eval_systolic_os 

pub fn eval_systolic_os(
    os: &SystolicOs,
    a: &[i64],
    w: &[i64],
    bits: u32,
) -> Vec<i64>

Reference int8 GEMM for the output-stationary tile: O[i][j] = Σ_t A[i][t]·W[t][j] (wrapping i64).