Derives c = √(K_s/ρ_s) as the substrate's maximum reorganisation rate — not a property of light specifically. A photon is a propagating organised substrate disturbance with no condensation to maintain, leaving all propagation budget for travel at c. Derives the minimum coherent photon energy E_min = 2.25 meV (λ = 88 μm). Below E_min, soliton propagation is impossible. Soliton persistence lengths: optical 11 AU, 100 MeV gamma rays 88.5 Gpc.