Primordial Phase Coherence and the Acoustic Universe: BAO, CMB, and the High-Coherence Limit of RCD (RCD Paper XXI)

We examine whether the observed primordial phase coherence — manifest in the CMB acoustic peaks and the BAO feature — is consistent with the high-coherence cosmological limit of RCD, where C approaches 1 at early times (Paper XIX). We show that: (i) RCD's linear perturbation sector is indistinguishable from LambdaCDM for these observables (Paper V), so acoustic phase coherence is automatically preserved; (ii) C ~ 1 provides a field-theoretic label for the adiabatic initial conditions without replacing inflation; (iii) no new signatures arise at the linear level. This paper establishes linear-level consistency, clarifies the interpretive role of C ~ 1, and identifies the late-time precision frontier where the degeneracy can in principle be broken.