A 28-year-old woman presents to the emergency department with palpitations and dyspnea for 2 hours. She reports a history of similar episodes since childhood, often triggered by emotional stress or caffeine. Vital signs: HR 180 bpm, BP 110/70 mmHg, RR 20/min. Physical examination reveals no structural abnormalities. 12-lead ECG shows a regular narrow-complex tachycardia with a rate of 180 bpm; P waves are buried within or immediately after the QRS complex. Adenosine 6 mg IV is administered, resulting in immediate termination of the tachycardia and restoration of normal sinus rhythm. What is the most likely diagnosis?