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 geometric and admissibility-driven rather than experimental. Preliminary version made publicly available on Zenodo. This manuscript has not yet undergone peer review and may be subject to revision.