Function auc_segment 

pub fn auc_segment(
    t1: f64,
    c1: f64,
    t2: f64,
    c2: f64,
    method: &AUCMethod,
) -> Result<f64, ObservationError>

Calculate AUC for a single segment between two time points

For AUCMethod::LinLog, this falls back to linear because segment-level calculation cannot know Tmax context. Use auc or auc_segment_with_tmax for proper LinLog handling.