A 32-year-old man from rural Bihar presents with a 3-week history of bloody diarrhea with mucus, abdominal cramping, and tenesmus. He denies fever. Stool microscopy reveals trophozoites with ingested RBCs. Stool culture is negative for bacterial pathogens. Colonoscopy shows flask-shaped ulcers in the cecum and ascending colon with intact intervening mucosa. What is the most likely diagnosis?