Ξ-Decoupling at z^† ≈ 1.7: Retained Geometry Without Continued Dynamical Growth

This preprint proposes that several late-time cosmological anomalies arise from a single coherence-loss transition in the dynamical state of the Universe. The framework introduces a hidden mismatch field χ between a retained internal structural sector and the sector governing continued dynamical growth. When the mismatch exceeds a critical threshold, χ > χ_c ⇒ Ξ → 0, S_int ≠ 0 the coherent growth node disappears while retained geometry remains finite. The model therefore predicts a new post-dynamical regime characterised simultaneously by: • γ ≈ 0.64 • a plateau in fσ8 • L/G > 1 • a common transition redshift z^† ≈ 1.7 The manuscript derives a falsifiable Ξ-decoupling theorem, a nonlinear cusp-catastrophe transition with hysteresis, and a unified explanation for the lensing-growth mismatch, structural saturation, and the emergence of retained geometry without continued coherent growth.