A 48-year-old male with a 20-year history of duodenal bulb ulcer. Symptoms worsened over the past 2 months, with pain occurring both before and after meals. For the past half month, frequent vomiting of large amounts of retained food with foul odor was noted. Gastroscopy revealed pyloric obstruction and a 2.5cm × 2.5cm ulcer on the lesser curvature of the stomach, with central deep excavation, dirty necrotic base, smooth margins, slightly increased firmness, minimal bleeding on biopsy, and reduced peristalsis. The most likely diagnosis is: