Carnot geodesics with Hölder controls

We construct an explicit length-minimizing curve in a polynomial rank-two sub-Riemannian structure on $\R^4$. Its arclength parametrization is $C^{1,1/2}$ and is not $C^{1,α}$ near an interior point for any $α>1/2$. Minimality is proved by an approximate calibration chosen separately for each compet