A 35-year-old male patient is undergoing elective laparoscopic cholecystectomy under general anesthesia. After induction with propofol and intubation with succinylcholine, the anesthesiologist begins maintenance with isoflurane at 1.2% end-tidal concentration. Intraoperatively, the patient develops unexplained tachycardia (HR 140/min), muscle rigidity, and a core temperature rise to 38.5°C. The end-tidal CO₂ has risen to 65 mmHg. What is the most appropriate immediate next step?