A 38-year-old woman from Mumbai presents with a 6-month history of progressive abdominal distension. She denies vaginal bleeding or constitutional symptoms. On examination, there is marked ascites and a large, smooth, firm abdominal mass. Serum CA-125 is 320 U/mL. Ultrasound reveals a large, predominantly cystic ovarian mass with thick septa and locules, and ascites. Histopathology shows columnar epithelium with mucin production, focal stratification, and mild nuclear atypia. What is the most likely diagnosis?