Function render_constraint_equation 

pub fn render_constraint_equation(prob: &NlProblem, k: usize) -> String

Render constraint k as a full relation, e.g. mass_in - mass_out = 0 or 0 <= T_reactor <= 500. Bounds outside ±1e19 are treated as infinite (AMPL’s convention), matching [TNLPAdapter]’s classifier.