Observational Constraints on a Phenomenological Nested Cosmology with Spatial Domain Transitions

We present an effective phenomenological cosmology framework incorporating Einstein–Cartan spin-torsion bounce dynamics (\(a_{\min} = 0.17\)), metric-continuous spatial domain boundaries (\([h_{ab}] = 0\)), and Chameleon scalar screening (\(M_{\text{scale}} = 3.5 \times 10^{17}\text{ GeV}\)). Using a reproducible, multi-messenger verification pipeline, we test the framework against 1,701 Pantheon+SH0ES Type Ia supernovae (full STAT+SYS covariance), eight DESI DR2 and DES Y6 Baryon Acoustic Oscillation (BAO) distance measurements, NASA Fermi-LAT Pass 8 diffuse gamma-ray templates, VLT/Keck quasar fine-structure spectroscopy, and CERN LHC Run 2 HEPData exclusion bounds. A profile likelihood scan over \(R_0 \in [50, 300]\text{ Mpc}\) identifies a local bulk-flow shell feature at R₀ = 52.5 Mpc (Δμ = -0.1501 ± 0.0242 mag) alongside a mild outer causal shell signature at R₀ = 120.7 Mpc (Δχ² = -4.78), yielding a joint SNe+BAO improvement of Δχ² = -37.80 (ΔAIC = -33.80, ΔBIC = -22.91). All electromagnetic coupling, and gamma-ray dipole bounds are satisfied. Accompanying Jupyter verification code is included in this deposit.