Dimensional reduction as a limit to boundary phase inference

This work identifies a structural limit to boundary phase inference at strongly asymmetric interfaces. When admissibility and boundary compatibility suppress normal propagating transport for the boundary-layer contribution, an exact boundary-induced dimensional reduction occurs, yielding a surface-confined phase observable despite vanishing normal energy flux. The resulting bulk-to-boundary phase mapping is structurally non-invertible: even in the ideal noiseless limit, boundary phase measurements cannot uniquely reconstruct the underlying bulk elastodynamic state. This limitation is geometricand admissibility-driven rather than experimental. This version adds a supplementary support document (“Support materials”) that provides a structured synopsis and contextual reorganization of material already presented in the main manuscript, including reuse of the existing schematicfigure for clarity. No assumptions, definitions, results, or conclusions of the main manuscript have been modified. Preliminary version made publicly available on Zenodo. This manuscript has not yet undergone peer review and may be subject to revision.