Function compute_step_quality 

pub fn compute_step_quality(
    current_cost: f64,
    new_cost: f64,
    predicted_reduction: f64,
) -> f64

Compute step quality ratio (actual vs predicted reduction).

Used by Levenberg-Marquardt and Dog Leg optimizers to evaluate whether a proposed step improved the objective function as predicted by the local quadratic model.

Returns ρ = actual_reduction / predicted_reduction, handling near-zero predicted reduction gracefully.