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| ∫ Lₘ²/R sqrt (-g) d⁴x. The central result is an exact decoupling theorem at recombination. In Entangled Relativity, the matter Lagrangian Lₘ 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 LEM = -1/4 F², not the fluid approximation Lₘ = -rhogamma. For thermal photons, the E B symmetry of free electrodynamics gives = 0. Together with non-relativistic baryons and electrons, this yields Lₘ = 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 Omegaₘ - Omegab ≈ 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 Omegacdm, while explicitly marking the full Cₗ computation as a future test.