A 68-year-old woman undergoes imaging for chronic pelvic pain and is found to have a large, multiloculated cystic ovarian mass measuring 18 cm. The mass is predominantly cystic with minimal solid component. Serum CA-125 is 45 U/mL (normal <35 U/mL, only mildly elevated). There is no ascites. Histopathology shows tall columnar mucinous epithelium lining the cyst, with areas of mucin extravasation into the stroma but no invasion of the ovarian capsule. What is the most likely diagnosis?