Statistical-arbitrage pairs trading is the canonical relative-value strategy. For each pair,
we compute the price ratio r_t = close_A / close_B, then track its 60-day rolling
Z-score. When |z| > 2 the pair is EXTENDED; |z| > 3 is
EXTREME — high-conviction mean-reversion entry. Half-life from
Ornstein-Uhlenbeck regression tells us how fast the spread typically reverts. Pairs are
uncorrelated with directional bets, so the strategy works in any regime
— bull, bear, slowdown, or expansion.