GRP Addendum — CMB Decoupling Theorem and Dark Matter Reinterpretation

This addendum develops the CMB-sector closure of the Geometric Relay Programme (GRP), built on the Entangled Relativity action of O. Minazzoli, S = -|C| ∫ L_m^2/R sqrt(-g) d^4x. The central result is an exact decoupling theorem at recombination. In Entangled Relativity, the matter Lagrangian L_m enters the action explicitly, so the usual perfect-fluid ambiguity becomes physical. This work argues that, for radiation, the correct object is the microscopic electromagnetic Lagrangian L_EM = -1/4 F^2, not the fluid approximation L_m = -rho_gamma. For thermal photons, the E <-> B symmetry of free electrodynamics gives <L_EM> = 0. Together with non-relativistic baryons and electrons, this yields L_m = T at z ≈ 1100, so Entangled Relativity reduces exactly to General Relativity at recombination. The addendum then identifies the relay tension R*(z), scaling as (1+z)^3 in the GRP cosmological mixed regime, with the quantity Omega_m - Omega_b ≈ 0.266 measured in ΛCDM as cold dark matter. In this interpretation, the CMB does not need to be modified: it measures the total matter density in the GR limit, while the geometric relay later accounts for galactic MOND-like behaviour through the same sector. The document also discusses the three CDM-like properties of the relay tension at recombination: matter-like scaling, non-oscillation in the Hubble-frozen regime, and CDM-like clustering through Memory Kernel filtering. A verification script is provided to check the numerical claims using physical constants and Planck 2018 parameters. This addendum does not claim to replace a full Boltzmann-code calculation. It identifies the structural decoupling mechanism and the relay reinterpretation of Omega_cdm, while explicitly marking the full C_l computation as a future test.