Where Lorentz Invariance Breaks Down: The Distinguishing Prediction wavevector k energy ω low k intermediate k → k_Planck Lorentz-invariant regime Planck-scale breakdown breakdown scale ω = ck (exact Lorentz) roton minimum the "dip" — unique to superfluid substrates maxon generic QG correction ω² = c²k² ± k³/M_Planck monotonic — no dip Substrate prediction (phonon → maxon → roton → recovery) Generic quantum gravity correction The roton-minimum signature is qualitatively different from any generic QG prediction. Observable in ultra-high-energy cosmic ray spectra or gamma-ray burst dispersion (Fermi-LAT, CTA). The substrate leaves a fingerprint that a simple Planck-scale cutoff does not.