Function two_arm_count_response 

pub fn two_arm_count_response(
    true_energies_ev: &[f64],
    incident_fluence_weights: &[f64],
    transmission: &[f64],
    detector_time_edges_us: &[f64],
    timing_offset_us: f64,
    response: &ResolutionFunction,
) -> Result<TwoArmCounts, CountsResponseError>

Apply one instrument response to the open-beam and sample arms separately.

incident_fluence_weights[j] is an expected neutron count integrated over the true-energy quadrature element represented by true_energies_ev[j], with detector efficiency already folded in (F_j = w_j ε(E_j) Φ(E_j) — see the module docs and pipeline-map R5·7). Probability outside detector_time_edges_us is intentionally not renormalized into the acquisition window; the expected counts lost outside it are reported per arm on the returned TwoArmCounts.

This function models only source fluence, detector efficiency (inside F_j), sample attenuation, and the instrument response. Detector/gamma/ scattering backgrounds and exposure normalization are separate physical terms and must be added by a higher layer that states their measurement location.