Five-variable weak Markus-Yamabe: graded compression and affine-Hessian obstructions

Unrefereed AI-assisted mathematical candidate. Two complete stated bigraded compression families built from a six-dimensional weak Markus-Yamabe example are excluded by exact polynomial certificates. A uniform Hessian-span theorem excludes constant Jacobian trace after invertible affine source-target changes for a specified family with arbitrary lower terms affine in three auxiliary variables. The global five-dimensional construction/nonexistence problem remains open. Exact replay is not proof-assistant formalisation or external peer review. Original prose/data are CC0-1.0; original code is MIT.