pub fn y_shape(x: f64) -> f64
Y-distortion (Sunyaev-Zeldovich) spectral shape: Y_SZ(x) = G_bb(x) · [x·coth(x/2) - 4]
This is the classic SZ spectral function; crosses zero at x ≈ 3.83. Reference: Zeldovich & Sunyaev (1969).