Constant PAIRED_TRIAL_SLOT 

pub const PAIRED_TRIAL_SLOT: u64 = 0;

The fixed trial-number slot paired (Pairing::Paired) seeds are derived from, in place of the real trial number.

Using a constant here is exactly what makes a paired replicate seed a pure function of (study_seed, replicate_index) — independent of which trial is being evaluated — and therefore identical across every trial. Its value is part of the stability commitment: changing it resamples every paired study.