A Duality-Invariant Extended Action for Nonlinear Partially Massless Spin Two

We construct a local covariant extended action for the full nonlinear spin-two partially massless Dirac-Born-Infeld equations of Cherney, Deser, Waldron and Zahariade. Two independent rank-two auxiliary fields pair with the constitutive divergence and the dual curvature divergence. All original equations follow from auxiliary variations, and every original solution lifts with zero auxiliary fields. The density is invariant off shell under the original infinitesimal nonlinear duality, extended by rotation of the auxiliary pair. Equality of physical degrees of freedom and a global finite-duality construction are not claimed. The deposit contains the manuscript PDF, LaTeX sources, Lean proofs and audit logs, and symbolic checks. The Lean development verifies continuous-integral projection and explicit constitutive calculations. Its geometric and analytic assumptions and the separately computed de Sitter counterexample are distinguished in the manuscript and theorem map.