Boundary Dominance in High-Dimensional Record Space: A DAGI/ESG Note on Causal Cuts, Horizons, and the Disappearing Bulk

This technical note imports a standard high-dimensional geometry fact into DAGI as a measure/accessibility principle, not as a proof of spacetime discreteness, holography, black-hole entropy, or gravity. In a side-2 hypercube, the largest inscribed unit ball occupies a fraction χ_d = π^(d/2)/(2^d Γ(d/2+1)), which decays super-exponentially with dimension. The proposed DAGI translation is that, as the effective number of independent record directions grows, a fixed low-distinction central sector becomes negligible relative to boundary-like sectors of distinguishable record assignments. The note defines an observer-indexed effective record dimension d_eff(O), a null-model boundary-dominance index BD_O, and a cut-aware diagnostic BD_cut that promotes only when high-dimensional reliable record structure coincides with positive active mass crossing a declared screen. The synthetic test suite stress-tests geometry sanity checks, effective-dimension calibration, record-cube benchmarks, causal-cone versus subset comparisons, screen-sensitive HOIC tests, soft-boundary leakage overlays, record-dephasing and eraser simulations, and threshold-recovery audits. The results are methodological: boundary dominance is a measure/accessibility diagnostic, not a dynamical leakage theorem or a gravitational derivation.