Function calculate_dense_times 

pub fn calculate_dense_times(
    start_time: f64,
    end_time: f64,
    obs_times: &[f64],
    idelta: f64,
) -> Vec<f64>

Generate a dense time grid for AUC calculations.

The grid contains the observation times plus regular points spaced idelta apart (in the model’s time units), sorted and deduplicated. A non-positive idelta disables the regular grid, leaving only the observation times.