A 13-year-old obese boy presents with a 6-week history of progressive left groin pain and limp. On examination, he has obligatory external rotation of the hip on flexion and limited internal rotation. AP hip X-ray is obtained. The line drawn along the superior cortex of the femoral neck (marked **C** in the diagram) fails to intersect the femoral head epiphysis, creating a characteristic gap. What is the most likely diagnosis and the immediate management priority?