A 52-yeаr-оld wоmаn (BMI 34 kg/m²) is 40 minutes intо а laparoscopic cholecystectomy under general anaesthesia with sevoflurane and remifentanil. The anaesthesiologist notices the end-tidal CO2 has risen to 74 mmHg despite unchanged ventilator settings. Examination reveals generalised muscle rigidity, jaw clenching, temperature 39.8°C, and heart rate 138 bpm. Arterial blood gas confirms mixed respiratory and metabolic acidosis. What is the most likely diagnosis?