A 34-year-old woman from Mumbai presents with secondary infertility of 2 years duration. She had one uncomplicated vaginal delivery 5 years ago. Her menstrual cycles are regular (28 days, normal flow), and she has no dysmenorrhea. On hysterosalpingography (HSG), the uterine cavity shows a filling defect in the fundal region, and contrast does not flow into the fallopian tubes bilaterally. Transvaginal ultrasound reveals a thin endometrium (5 mm in the mid-luteal phase) with multiple echogenic bands crossing the uterine cavity. What is the most likely diagnosis?