This paper develops four formal bridges that embed the BRISM interface model within the established Hilbert‑space framework of quantum mechanics. BRISM describes how real, normalized measurement statistics on the brane arise from complex bulk amplitudes through a phase‑neutral interface mapping, without modifying the standard formalism or introducing additional ontology. (i) U(1) symmetry it is not an added physical assumption but the structural completion of the measurement formalism.(iv) Spectral stability (new): Only quadratic density mappings remain compatible with positivity, locality, σ‑additivity, and phase neutrality across spectral components, making the Born rule a structural necessity of the interface rather than a postulate. Overall, the work reorganizes the standard framework internally, clarifying how observable statistics emerge from the structural properties of the bulk–brane interface. It extends the conceptual basis introduced in the author’s earlier BRISM papers (the foundational BRISM paper DOI: 10.5281/zenodo.18391944.
Swen Carlo Heinze (Thu,) studied this question.