Elimination of the GGHV Branch-(a,b) Normal Form at Degree Pair (72,108): A Certified Computational Audit — Layer Reduction, Top-Layer Classification, and Final Obstruction

This paper eliminates the GGHV branch-(a,b) normal form for the 2D Jacobian conjecture at degree pair (72,108). The main theorem proves that no polynomials P,Q in NewtonNF2 normal form satisfy [P,Q]=λx² with λ≠0, via a certified computational audit: (I) Newton polygon layer reduction to triangular systems E5...E1; (II) top-layer classification via a kernel-checked Lean 4 formalization (chartClassification_holds) giving five torus orbits over K5=ℚ[w]/(w^5-w^4+3w^3+3w^2+26); (III) exact K5 descent obstruction via 35 homogeneous quintic minors spanning K5[t1,t2]5, forcing t=0; (IV) vertex destruction giving b12,24=0, a contradiction. The Lean formalization (main_theorem) is independent of GGHV Proposition 4.3. Companion GitHub repository: https://github.com/brandonmccraryresearch-cloud/2D-Jacobian-Conjecture-Workspace/tree/main/branch_ab_v19 (artifact commit c74606710b3a6c77c52b8b6f945c913658ab4eec).