A 32-year-old male from rural Maharashtra is undergoing elective open reduction and internal fixation of a femur fracture. Fifteen minutes after induction with propofol and succinylcholine, the operating room team notices masseter muscle rigidity, followed by a rapid rise in end-tidal CO₂ (ETCO₂ from 35 to 60 mmHg within 5 minutes), core temperature rising at 1°C per minute, and dark-colored urine in the catheter. What is the most appropriate immediate next step in management?