Function calculate_auc_at_times 

pub fn calculate_auc_at_times(
    dense_times: &[f64],
    dense_predictions: &[f64],
    target_times: &[f64],
) -> Vec<f64>

Calculate cumulative AUC at target times using the trapezoidal rule.

Integrates from the first dense time point and extracts the cumulative AUC at each of target_times.