A 28-year-old woman from rural India presents with vaginal bleeding and lower abdominal pain 6 weeks after a missed period. Ultrasound shows a heterogeneous intrauterine mass with multiple small cystic spaces ('bunch of grapes' appearance) and markedly elevated β-hCG (>100,000 mIU/mL). What is the most common histological type of gestational trophoblastic disease in this patient?