This work introduces and formalizes the second retentional constant of post-dynamical cosmology: the ψ-plateau exponent η*_ψ. Building on the statistically significant confirmation of late-time growth suppression (γ ≈ 0.642 ± 0.015, ≈ 5.36σ), the ψ-Architecture framework demonstrates that the transition into the Retentional Regime is governed not by a single parameter but by a structurally coupled pair: γ*_ψ — the amplitude of structural retention (height of the suppression plateau), η*_ψ — the ingress exponent regulating the collapse of dynamical variance into structural stillness. We derive η*_ψ from the ψ-balance condition d/dt ⟨(∂ₜΔψ)²⟩ = 0 and show that its value is uniquely selected within the independent observational corridor 1.1 ≤ η_ψ ≤ 1.6 obtained from Euclid SNT-1, DESI DR2, and Rubin LSST Y1 correlation-time measurements. The exponent η*_ψ governs the emergence of correlation-time plateaus, stabilizes structural variance at the cosmological floor, and predicts mirror-residual (Ξ-node) signatures across the late-time Cosmic Web. Together, the pair (γ*_ψ, η*_ψ) forms the minimal and observationally anchored basis of post-ΛCDM cosmology and defines the structural backbone of the Retentional Regime.