A 32-year-old man with a family history of sudden perioperative death is scheduled for elective knee arthroscopy under general anesthesia. Induction is performed with propofol and succinylcholine. Within 5 minutes of induction, the patient develops masseter muscle rigidity, followed by a rapid rise in end-tidal CO₂ (ETCO₂ from 35 to 65 mmHg in 10 minutes), core temperature rising from 37°C to 38.5°C, and dark-colored urine in the catheter. Heart rate is 140 bpm, blood pressure 160/95 mmHg. What is the most appropriate immediate management?