A 58-year-old woman presents with a 2-month history of abdominal discomfort and dark stools. Physical examination shows pallor but no evidence of jaundice. Laboratory studies disclose a microcytic, hypochromic anemia, with a hemoglobin level of 6.7 g/dL. A barium swallow radiograph reveals a "leather bottle" appearance of the stomach. Microscopic examination shows diffusely infiltrating malignant cells, many of which are "signet ring" cells, in the stomach wall. Which of the following is the most likely diagnosis?