A 3-month-old infant with congenital talipes equinovarus has been undergoing Ponseti casting for 6 weeks. The parents report that the foot position has improved, but the child now presents with a stiff, plantarflexed foot that does not respond to further casting. The treating orthopedist suspects an atypical or complex CTEV variant. Which investigation would be most appropriate to assess the soft tissue anatomy and plan surgical intervention?