Function joint_marginal_entropy_bytes 

pub fn joint_marginal_entropy_bytes(x: &[u8], y: &[u8]) -> f64

Compute joint marginal entropy H(X,Y) = −Σ p(x,y) log₂ p(x,y) in bits/symbol-pair.

Uses a direct histogram of (x_i, y_i) pairs. This is the exact first-order joint entropy, matching the spec.md definition.