We give a mechanism-level account of why the -sector of Lambda Convergence Universal Field Theory (LUFT) admits a single invariant propagation cone. Starting from the stability functional I on configuration space C as axiomatized in the companion Layer-0 paper, we characterize a propagation constitutive tensor chiₙu on the emergent structure M₄₅₅, postulate its variational origin (stated as an explicit principle), and show that the Skewon part then vanishes. Explicit Fresnel analysis of the isotropic principal part via the Tamm--Rubilar quartic yields a perfect-square factorization: a single null cone of Lorentzian signature, shared by both polarizations --- zero vacuum birefringence. The cone slope is the constitutive ratio _/_; what fixes this ratio in LUFT terms is an open problem and is stated as such. The identification c² = A/B of earlier versions of this paper is retracted: per Spine v3. 0. 0 the ratio of the structural norms is an exact quadrature constant of the sublevel geometry (2 or 8 by reading choice), flat in the coherence parameter and invariant under the kinetic coefficients --- it carries no kinematic content. The superseded machinery is preserved, reclassified, in Appendix app: quadrature, together with the in-paper numerical finding that foreshadowed the diagnosis. Equality of the -sector cone with the -sector dispersion cone and the gravity-chain bundle cone is the cone-universality conjecture of the Spine, empirically constrained at the 10^-15 level by GW170817-class observations. This is not a numerical derivation of the value of c: it is a mechanism-level explanation of the existence, uniqueness, and Lorentzian character of the cone, with physical dimensions arising only through observer embedding. Version note (v3. 0. 0, June 2026; uploaded 2026-07-02): major claim restructuring. The central identification c2 = A/B of v2. x is retracted — per the spine norm-scaling result, A/B is an exact quadrature constant with no kinematic content. The single invariant cone is rebuilt via a pre-metric Fresnel analysis; the constitutive slope (alphachi/betachi) is recorded as an open gap. The earlier title and abstract of this record are superseded. Full changelog in the PDF.
Ilja Schots (Thu,) studied this question.