A 35-year-old man with a history of palpitations since childhood presents with a 4-hour episode of rapid palpitations. He reports that during the attack, he feels a sudden "thump" in his chest followed by rapid beating. Vital signs: HR 200 bpm, BP 105/65 mmHg, RR 20/min. ECG during the tachycardia shows a narrow-complex tachycardia with clearly visible P waves occurring **after the QRS complex with an RP interval of 80 ms**. The RP interval is **shorter than the PR interval**. Carotid sinus massage slows the tachycardia gradually but does not terminate it. What is the most likely diagnosis?