Function calculate_interval_auc_per_observation 

pub fn calculate_interval_auc_per_observation(
    subject: &Subject,
    dense_times: &[f64],
    dense_predictions: &[f64],
    obs_times: &[f64],
) -> Vec<f64>

Calculate interval AUC for each observation independently.

For each observation at time t, integrates from the last dose before t to t (e.g. dosing-interval AUCτ at steady state).