A First-Principles Derivation of DBI Non-Gaussianity: Predicting GUT-Scale Brane Tension for SPHEREx

Version 2.1 - Major phenomenological revision. This version replaces the speculative treatment in v1 (June 5, 2026) with a standard DBI inflation analysis [1,2]. Key updates:1. Derives brane tension scale τ^{1/4} = 5.5e17 GeV from f_NL = 8 using Silverstein & Tong 2004.2. Adds Figure 1 showing falsifiable prediction for SPHEREx [3]. Gray region excluded by Planck 2018.3. Section 2 explicitly lists 3 open questions for UV completion: bulk photon dynamics, reheating efficiency, and origin of τ. v1 remains available for transparency and documents the evolution from initial concept to testable phenomenology. References:[1] Silverstein & Tong, Phys. Rev. D 70, 103505 (2004)[2] Alishahiha, Silverstein & Tong, Phys. Rev. D 70, 123505 (2004) [3] Doré et al., arXiv:2103.11888 (2021)