A 28-year-old female with a family history of unexplained perioperative death is scheduled for elective cesarean section. During induction with propofol and intubation, anesthesia is maintained with nitrous oxide and oxygen. Thirty minutes after skin incision, the obstetrician notes generalized muscle rigidity, jaw clenching, and difficulty with surgical access. Core temperature is 38.1°C, ETCO₂ is 52 mmHg, and the patient's urine is dark. Serum potassium is 6.2 mEq/L. What is the most likely diagnosis and the immediate next step?