A 38-year-old woman from rural Bihar presents with progressive bone pain, muscle weakness, and difficulty climbing stairs for the past 8 months. She reports minimal sun exposure due to cultural practice of purdah. On examination, she has proximal muscle weakness (grade 4/5), bone tenderness over the tibia and femur, and a waddling gait. Serum calcium is 7.2 mg/dL, phosphate 2.8 mg/dL, alkaline phosphatase 120 IU/L (elevated), and 25-hydroxyvitamin D is 12 ng/mL. X-ray shows loss of sharp metaphyseal margins and Looser's zones in the femur. What is the most likely diagnosis?