A Geometric Origin for the Cosmological Constant from CPT-Symmetric Branes: Framework, Flux Stabilization, and Topological Quantization of 𝜌 Λ

We present the Dual-Brane Quantum Tunneling Model (DBQTM), a geometrically motivated framework for the cosmological constant based on a CPT-symmetric two-brane configuration in a Randall--Sundrum (RS) background with Goldberger--Wise (GW) stabilization. The observed vacuum energy arises from the inter-brane mixing suppression ρ_Λ = k⁴ · exp(-ngeo · kd), with k = 0.0869\,MPl, ngeo = 0.720, and kd ≈ 375.The 122-order-of-magnitude hierarchy between the natural and observed vacuum energy scales is recast as a moderately large geometric product n · kd ≈ 270. We derive the Binetruy--Deffayet--Langlois junction conditions self-consistently, showing that the physical brane tension λₑff = VA/2 ≈ 0.09\,MPl⁴ exceeds ρcrit by 10¹²⁰, making w(z) = -1 + O(10⁻¹¹⁹). A no-go theorem establishes that vacuum energy suppression and observable BDL corrections are mutually exclusive within the RS framework. To address the question of why kd ≈ 375, we explore four flux-stabilization mechanisms. The most natural is a Klebanov--Strassler embedding, where the brane separation is determined directly by quantized 3-form fluxes: kd = 2π K/(gₛ M). With gₛ = 1/5, $K = 203$, $M = 17$, we obtain $kd = 375.14$ and ρ_Λ within 2% of the observed value. The cosmological constant becomes a topological invariant --- a consequence of two integers characterizing the flux threading the compactification manifold. 2.0: Adds parameter scans (Appendix~A),flux vacuum stability via CDL/HM instantons (Appendix~B), andcritical comparison with six competing frameworks includingranked limitations (Section~5). Core physics unchanged.