A 32-year-old woman is admitted 20 minutes after intentional ingestion of potassium cyanide (KCN) in a suicide attempt. She is unconscious, with fixed dilated pupils, generalized seizures, and profound bradycardia (HR 38/min). Skin is cyanotic despite supplemental oxygen at 15 L/min via non-rebreather mask. Blood pressure is 78/52 mmHg. Venous blood gas shows SvO₂ (mixed venous oxygen saturation) of 88% (normal 70–75%). Which antidote should be administered first as part of the immediate management?