A 38-year-old woman undergoing open abdominal hysterectomy receives intraoperative fentanyl 15 mcg/kg IV. Induction and intubation proceed smoothly. Thirty minutes into surgery, the anesthesiologist notices the patient's end-tidal CO₂ has risen to 58 mmHg (baseline 38 mmHg), temperature is 38.8°C and rising, jaw muscles are rigid, and the patient is profusely diaphoretic. Heart rate is 145/min and blood pressure is 165/95 mmHg. What is the most likely diagnosis?