Matter Coupling and Sound-Speed Bound in the Quasi-Static Dust Limit

We derive the full second-order action for scalar perturbations in the disformal RCD galactic sector including pressureless dust. We prove the kinetic coefficient receives a positive-definite matter correction Delta_Q > 0, yielding c^2_s = c^2_{s,QS}/(1+epsilon) with epsilon > 0. The critical density for subluminality is rho_crit ~ rho_{crit,cosmo}, implying epsilon >> 1 throughout any gravitationally bound structure. In the solar neighborhood, epsilon ~ 3e5 and c_s/c ~ 0.002.