Dynamical Stabilization of the RCD Hilltop via Coherential Vacuum Backreaction RCD Hilltop Dark Energy Series — Paper III

Papers I–II established a no-go theorem: no local kinetic modification can stabilize a hilltop with V’’(C₀) < 0. This paper identifies the minimal evasion within the RCD framework by lifting the decoupling approximation to include leading-order backreaction from the sub-coherential microstructure. We work in the semiclassi- cal Markovian approximation of a Caldeira-Leggett bath coupled to the coherence field. Integrating out the bath produces a static Coleman-Weinberg correction (α₀) and a dynamical Markovian piece (Λ = αγ₀/Γ_eff). Within the class of Markovian Caldeira-Leggett completions considered, α₀ + Λ > v is necessary and sufficient for linear stabilization. We do not claim a UV derivation of the effective coefficients; we identify the minimal ingredient, declare the regime of validity (including the weak- backreaction parameter ε ≪ 1), and verify numerically. Non-adiabatic dynamics (fi- nite Γ/H) demonstrates that the stabilization is an active dynamical process with a characteristic three-phase transient, not a static reparametrization.