Function empirical_cross_entropy_bytes 

pub fn empirical_cross_entropy_bytes(test_data: &[u8], train_data: &[u8]) -> f64

Empirical (zero-order, IID) cross-entropy H₀_q(p) = -Σ p(x) log₂ q(x) between the byte histograms of test_data (treated as p) and train_data (treated as q).