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. This version consolidates the statement and proof of the structural non-invertibility result. The proof is now given explicitly in Appendix A. No technical results have been changed. Preliminary version made publicly available on Zenodo. This manuscript has not yet undergone peer review and may be subject to revision.