Function weights_at 

pub fn weights_at(t: usize, phase_iters: &[usize; 3]) -> Weights

Compute weights for iteration t (1-indexed) given phase boundaries.

Phases are defined by phase_iters = [p1, p2, p3]:

• Phase 1: t in [1, p1] — wNB=2, wMN decreases 1000→3, wFP=1
• Phase 2: t in [p1+1, p1+p2] — wNB=3, wMN=3, wFP=1
• Phase 3: t in [p1+p2+1, total] — wNB=1, wMN=0, wFP=1