ψ-Retention Cosmology vs. Dark Energy: A Structural Reinterpretation of Cosmic Acceleration

This paper presents a structural alternative to dynamical dark energy within the ψ-Retention (Λψ) framework. Rather than interpreting late-time cosmic acceleration as the effect of a fluid with negative pressure or an additional dynamical field, ψ-Retention proposes that the Universe enters a retentive regime in which structure growth saturates at a geometric stability threshold. The framework preserves the Einstein metric sector (η = 1), introduces no new propagating degrees of freedom, and explains a coherent suite of late-time anomalies — including the S₈ tension, the weak ISW signal, the low-ℓ CMB deficit, the Planck A_L anomaly, and scale-invariant void lensing suppression — through a single non-increasing structural attenuation factor R(a). The paper contrasts ψ-Retention with dynamical dark-energy models, formulates the conceptual and observational differences, and identifies falsifiable predictions testable by Euclid DR1. This article constitutes Paper 4 of Volume IV of the ψ-Architecture Research Series.