Function gestalt_qualifies 

pub fn gestalt_qualifies(
    a: &[char],
    b: &[char],
    sam_b: &Sam,
    threshold: f64,
) -> bool

Threshold-aware exact decision: does RO(a,b) ≥ threshold? Computes the matched total M with two-sided early-exit — accept the instant M ≥ need, reject the instant the upper bound M + Σ min(window lengths) < need. Exact (the bound never drops a qualifying pair), and dissimilar pairs abort long before the full decomposition. need = ⌈threshold·(|a|+|b|)/2⌉.