Function complex_matmul 

pub fn complex_matmul(
    a: &[ComplexF64],
    b: &[ComplexF64],
    out: &mut [ComplexF64],
    m: usize,
    k: usize,
    n: usize,
)

Complex matrix multiply: C[m,n] = A[m,k] × B[k,n] (fixed-sequence).

Each element C[i,j] = Σ_p A[i,p] * B[p,j] with BinnedAccumulator.