Function mu_from_photon_injection

pub fn mu_from_photon_injection(
    x_inj: f64,
    z_h: f64,
    delta_n_over_n: f64,
) -> f64

Compute μ from monochromatic photon injection at frequency x_inj.

μ = α_ρ × x_inj × (3/κ_c) × J*(z_h) × J_μ(z_h) × [1 − P_s × x₀/x_inj] × ΔN/N

Uses the universal J_μ(z) visibility function (same as heat injection).

Sign behavior:

• x_inj > x₀ and P_s ≈ 1: μ > 0 (energy-dominated)
• x_inj < x₀ and P_s ≈ 1: μ < 0 (number-dominated, negative μ!)
• P_s ≈ 0 (soft photons absorbed): μ > 0 always (pure energy injection)

Reference: Chluba (2015), Eq. C7