On the Boundary Rigidity of the Planar Jacobian Conjecture Below Degree 125: A Computational Audit of the GGHV (72, 108) Candidate

New in this version (Supplement, September 23, 2026): adds the supplemental companion paper "On the Cancellation Polynomial Q²−P³ in the Planar Jacobian Conjecture: Branchwise Valuations, the Bézout Pincer, and the [38,179] Degree Window — A Supplemental Audit of the GGHV (72,108) Candidate" (19 pages, plus its full XeLaTeX source archive). The supplement records the complete Q1 = Q²−P³ program of September 20–23, 2026: the verified [38,179] degree window, the exact C-adic identity, the corrected branchwise valuations, the corrected Bézout pincer (N ≤ 59.5D), and the projective normalization — concluding Case (c)/Gate 7 remains underdetermined. Second edition of a computational audit of the degree-(72,108) Newton-polygon candidate (GGHV Case (c)) for the planar Jacobian conjecture. Reports a Lean 4 machine-checked valuative-properness predicate for the identity, one elementary triangular automorphism, and arbitrary GL_2(k)-linear maps; refutation of four local elimination shortcuts; a corrected Newton-Puiseux branch census at the two points at infinity; a capped corner-transfer sieve (48 negative-weight modes, degree-71 centralizer kernel, 264/408 frozen full-lattice modes, rank-2 linearized corner composite); and the cancellation identity {P, Q^2-P^3} = 2Q with deg(Q^2-P^3) >= 38. Conclusion: Case (c) and Gate 7 remain underdetermined. Companion file jc2d_computational_verification_files.zip contains all Python/Singular scripts, run logs, the CASE_C_RESOLUTION.md ledger, and the Lean 4 project sources; jc125_paper_source_files.zip contains the full XeLaTeX source.