Function visibility_j_y

pub fn visibility_j_y(z: f64) -> f64

y-distortion branching ratio: fraction of energy going into y-type distortion.

J_y(z) = [1 + ((1+z)/6.0×10⁴)^{2.58}]^{−1}

Functional form from Chluba (2013), arXiv:1304.6120, Eq. 5. The denominator constant here (6.0×10⁴) comes from the updated refit in Arsenadze et al. (2025), arXiv:2502.11432; the original Chluba 2013 value was 5.9×10⁴, a ~1–2 % shift across the transition region. We keep 6.0×10⁴ for consistency with the J_μ/(1-J_μ) transition scale and with CosmoTherm GF tables. Approaches 1 for z ≪ 6×10⁴ and 0 for z ≫ 6×10⁴. Note: J_y ≠ (1 − J_μ) in the transition region; using the independent fit gives better spectral agreement with PDE results.