A 4-year-old boy from rural Maharashtra presents with his mother to the pediatric outpatient clinic with a 3-week history of abdominal distension and palpable mass in the left flank. On examination, the mass is firm, non-tender, and does not cross the midline. Vital signs are stable. Abdominal ultrasound reveals a large heterogeneous mass arising from the left kidney with areas of cystic change and calcification. Serum alpha-fetoprotein (AFP) is normal, and urinary catecholamine metabolites (VMA and HVA) are within normal limits. What is the most likely diagnosis?