Function float_int_class 

pub fn float_int_class(f: f64) -> Option<i64>

The §5.4 canonical numeric class: Some(i) iff Float(f) is exactly equal to Int(i) under compare_int_f64 — finite, integral, in i64 range, and not -0.0 (which total_cmp orders below +0.0/Int(0) and so keeps float identity). Joint with the exact comparator this makes numeric equality an equivalence relation a Hash + Eq key can represent: Float(2^53) shares Int(2^53)’s class, while Int(2^53 + 1) keeps all 64 bits.