ψ-Architecture Research Series, Vol. IV — ψ-Retention Cosmology: A Structural Framework for Late-Time Stability Beyond Dynamical Suppression

This article constitutes Part II of the ψ-Architecture Research Series, Volume IV, presenting the structural and observational formulation of ψ-Retention Cosmology (Λψ). The framework interprets late-time cosmic evolution as a transition from dynamical amplification to structural stability, producing a characteristic growth plateau, suppressed S₈, weak ISW, null gravitational slip, and scale-invariant lensing suppression. The paper provides the structural layer of the Λψ corpus, complementing the companion technical work containing the full Lagrangian, retentive functional \mathcal{I}_\psi, and the universal stability threshold \mathcal{R}_\star = 0.512. The framework is fully falsifiable, GR-compatible, and tailored for Euclid DR1 (2026), offering a minimal retentive mechanism that unifies multiple independent cosmological anomalies without introducing new fields or modifying gravity.