Function y_from_heating

pub fn y_from_heating<F>(dq_dz: F, z_min: f64, z_max: f64, n_z: usize) -> f64
where
    F: Fn(f64) -> f64,

Extract y parameter from the Green’s function approximation.

y = (1/4) ∫ J_y(z) · d(Δρ/ρ)/dz dz

Uses the independently fitted J_y visibility function (Arsenadze et al. 2025), which gives better agreement with PDE results than (1 − J_μ).

Calls mu_y_from_heating internally and returns the y component. For simultaneous μ and y, use mu_y_from_heating directly.