A 28-year-old man is brought to the emergency department 15 minutes after ingesting cyanide salt in a suicide attempt. He is conscious but agitated, with a respiratory rate of 32/min and heart rate of 118/min. Arterial blood gas shows pH 7.28, PaCO₂ 32 mmHg, HCO₃⁻ 14 mEq/L, and PaO₂ 95 mmHg on room air. Venous oxygen saturation is 85%. What is the most likely mechanism of cyanide toxicity in this patient?