A 32-year-old Indian woman presents with severe dysmenorrhea and chronic pelvic pain for the past 3 years. She reports that pain begins 1–2 days before menstruation and persists throughout her cycle, worsening during menses. On examination, the uterus is uniformly enlarged, tender, and boggy. Transvaginal ultrasound shows a heterogeneous myometrium with ill-defined junctional zones and small cystic spaces within the myometrium. Her serum CA-125 is mildly elevated at 42 U/mL. What is the most likely diagnosis?