A 58-year-old postmenopausal woman from Delhi presents with progressive abdominal distension, early satiety, and vague lower abdominal pain for 3 months. On examination, she has ascites and a fixed left adnexal mass. CA-125 is 850 U/mL (normal <35). Transvaginal ultrasound shows a complex left ovarian mass with internal septations and free fluid in the pelvis. CT abdomen reveals omental caking and peritoneal nodules. What is the most likely stage of her ovarian cancer?