Lambda Convergence Universal Field Theory: The Emergence of Light — A Pre-Metric Mechanism for the Single Invariant Cone

We give a mechanism-level account of why the ν-sector of Lambda Convergence Universal Field Theory (LUFT) admits a single invariant propagation cone. Starting from the stability functional I[Φ] on configuration space C as axiomatized in the companion Layer-0 paper, we characterize a propagation constitutive tensor chiₙu on the emergent structure Meff, 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 α_χ/β_χ; 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 The superseded machinery is preserved, reclassified, in Appendix {app:quadrature}, together with the in-paper numerical finding that foreshadowed the diagnosis.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 ν-sector cone with the Φ-sector dispersion cone ακ 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 (alpha_chi/beta_chi) is recorded as an open gap. The earlier title and abstract of this record are superseded. Full changelog in the PDF.