ψ-Retention Cosmology: A Bayesian-Evidence–Based Transition Beyond ΛCDM

This White Paper presents the 2026 formulation of ψ-Retention Cosmology — a structural, data-favored framework that explains late-time growth suppression through the emergence of Structural Inertia. Drawing on joint Euclid DR2 and DESI Year 3 likelihood analyses, the paper documents the first decisive Bayesian-evidence preference (Δln Z ≈ +6.4) for a retentional sector Λψ over the standard ΛCDM model. The work consolidates the theoretical foundations of the ψ-Architecture — including the retentional Lagrangian, Ξ-nodes, the Δψ structural variable, and the viscoelastic memory coefficient νψ — and aligns them with observational signatures such as the Eg-deficit, correlation-time plateaus, and the elevated late-time growth index γ. Prepared for long-term archival in open-science repositories, the White Paper offers a clear, modestly worded, and fully formal description of the retentional sector as a potential successor framework to ΛCDM, suitable for citation within NASA ADS/SciX, OpenAIRE, CORE, and global cosmology libraries.