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 $\nu$-sector of Lambda Convergence Universal Field Theory (LUFT) admits a single invariant propagation cone. Starting from the stability functional $I[\Phi]$ on configuration space $C$ as axiomatized in the companion Layer-0 paper, we characterize a propagation constitutive tensor $chi_nu$ on the emergent structure $\mathcal{M}_{\mathrm{eff}}$, 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 $\alpha_\chi/\beta_\chi$; what fixes this ratio in LUFT terms is an open problem and is stated as such. The identification $c^2 = A/B$ 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 The superseded machinery is preserved, reclassified, in Appendix \ref{app:quadrature}, together with the in-paper numerical finding that foreshadowed the diagnosis. Equality of the $\nu$-sector cone with the $\Phi$-sector dispersion cone $\alpha\kappa$ 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 The earlier title and abstract of this record are superseded.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.