Function py_ph_analyze_crosstalk_pairs 

pub(crate) fn py_ph_analyze_crosstalk_pairs<'py>(
    py: Python<'py>,
    pairs_a: Vec<usize>,
    pairs_b: Vec<usize>,
    gaps_nm: Vec<f64>,
    lengths_um: Vec<f64>,
    wavelength_nm: f64,
    core_index: f64,
    cladding_index: f64,
) -> PyResult<Py<PyAny>>

Per-pair geometric crosstalk for arbitrary waveguide geometry. pairs_a[i], pairs_b[i], gaps_nm[i], lengths_um[i] describe pair i. Evaluated in parallel via Rayon — the O(N²) analysis path.