For each pair (A, B), we compute the price ratio r_t = A/B, then the
rolling 60-day mean and standard deviation. The current spread Z-score
z = (r_now − μ_60d) / σ_60d measures how stretched the relationship is.
When |z| ≥ 2, the pair is mean-reversion candidate (EXTENDED). When
|z| ≥ 3, it's EXTREME — historically a high-probability fade.
Half-life estimates how many trading days for the spread to
revert (Ornstein-Uhlenbeck). R:R is the ratio of expected reversion
to a 1-σ stop. 252d correlation validates the pair is still cointegrated.