We identify the quantum-classical boundary exactly: it is the boundary of the Feigenbaum universality class. The Universal Cascade Theory proves that three conditions — C₁ (dissipative boundedness), C₂ (non-degenerate quadratic fold), C₃ (transversal spectral crossing) — are necessary and sufficient for cascade structure with constants δ = 4.66920160… and α = 2.50290787…. The linear Schrödinger equation categorically fails C₂ and does not cascade. The quantum Kerr oscillator satisfies C₁–C₃ and exhibits a quantum phase transition at γˣ = 1.1838, a quantum tunneling correction Δγ = 0.738, and a Whisper scaling exponent β = −δ = −4.669… — all derived without free parameters. The Born rule |ψ|² is the unique probability measure invariant under cascade renormalization, forcing p = 2 uniquely. The boundary is not a matter of decoherence rate, scale, or interpretive convention. It is a universality class boundary, sharp and structural.
Lucian Randolph (Fri,) studied this question.