Function noise_seed 

pub fn noise_seed(seed: u64, number: u64) -> u64

A per-trial noise seed, so replicated curves get independent noise from one study seed.

The driver derives noise_seed(study_seed, trial_number) for each trial and hands it to LearningCurve::value_at; two trials of the same noisy curve therefore share a trend but not a realization, which is exactly “differ only by noise”.