ψ-Retention Cosmology: The Structural Stability Window (SSW-2026) A Retentive Effective Field Framework for Late-Time Cosmic Persistence

This work presents a strengthened formulation of ψ-Retention Cosmology, introducing the Structural Stability Window (SSW-2026) as a mathematically rigorous, testable framework for late-time cosmic persistence. We derive the retentive Lagrangian on a standard GR (FRW) background, establish the Δψ→0 transition, define the γψ retentive growth index, and provide a formal convergence proposition for the equilibrium state Δψ = (β/2α)Ξ. The framework predicts a stability plateau at k ≈ 1–5 h/Mpc, characterized by suppressed nonlinear evolution, correlation-time plateaus, and stabilization of gravitational potentials. These effects arise naturally from the retentive potential Λψ and offer a clear observational target for forthcoming Euclid DR1, DESI, and JWST analyses. The article includes full dimensional analysis, stability conditions, and operational tests required for empirical validation. ψ-Retention Cosmology is formulated as an effective structural layer consistent with GR, providing an interpretative extension rather than a modification of gravity. This submission is part of the ψ-Architecture Research Series (2026).