A Gated Effective Density Component near ΛCDM: Candidate Higher-Dimensional Interpretation, Benchmarks, and CLASS-Backed Reproducibility with Independent-Data Compatibility

We define a small phenomenological extension of the standard ΛCDM cosmological background by introducing an additional effective energy-density component modulated by a Hubble-threshold gate. The component reduces exactly to ΛCDM when its amplitude vanishes and preserves the present-day normalization. We specify the background formulation, the gate, two frozen benchmarks (S_safe, R2_main), candidate mechanisms, and explicit falsifiable predictions. The model is implemented in the Boltzmann code CLASS with audited background-level reproducibility, and the first-order (P1) perturbation sector is now implemented with a phantom-divide-safe formulation (a regularized momentum variable u_HD=(1+w_eff)theta_HD). Against Planck plik-lite, an information-criterion evaluation referenced to a refit ΛCDM baseline finds no statistical preference for the component (Delta AIC ~ +9). When confronted with independent data — DESI 2024 BAO, Pantheon/Pantheon+ supernovae, DES and SDSS large-scale-structure splits, and CMB A_planck nuisance variants — the component remains compatible at fixed-point evaluation, with no strong tension in any completed split. This record is a priority and reproducibility snapshot of a testable candidate. It is NOT a detection, NOT a claim of statistical preference over ΛCDM, NOT a confirmation of higher-dimensional physics, and NOT a claim of improvement in structure-growth (sigma8, S8); compatibility with independent data is a compatibility statement only, not a claim of preference or support from any dataset. Convergence of production MCMC and posterior inference, a full Planck likelihood beyond the plik-lite route, a formal Bayesian evidence treatment with controlled prior-volume sensitivity, per-dataset refits of the independent data, the shear sector beyond the zero-shear P1 prototype, and independent external reproduction remain as future work.