Linear Scalar Perturbations in the Quasi-Static Regime

We analyze RCD linear scalar perturbations in the quasi-static regime. The homogeneous cosmological background forces all EFT modification parameters to unity: mu = eta = Sigma = 1. RCD is indistinguishable from LCDM at linear scales, with Delta(f*sigma_8)/f*sigma_8 < 1e-6. Modifications activate only in the non-linear regime where coherence gradients become non-negligible. This linear safety is a structural consequence of the action, not a tuning.