A 62-year-old woman from Delhi presents with syncope during exertion while climbing stairs 2 days ago. She reports progressive exertional dyspnea for 6 months and chest discomfort on effort. On examination, blood pressure is 155/65 mmHg with a narrow pulse pressure, pulse 72/min regular. Carotid pulse is slow-rising and diminished. A systolic ejection murmur is heard at the right upper sternal border radiating to both carotids, with a single S2. Apex beat is displaced laterally. Echocardiography shows a peak aortic valve gradient of 65 mmHg, aortic valve area 0.8 cm², and left ventricular wall thickness of 18 mm. What is the most likely diagnosis?