A 52-year-old Indian man presents with a 6-week history of progressive dyspnea, cough, and chest pain. Chest X-ray shows a large anterior mediastinal mass with pleural effusion. CT chest confirms a 10 cm × 8 cm mass in the anterior mediastinum with compression of the superior vena cava. Pleural fluid analysis shows LDH 800 IU/L and protein 4.2 g/dL. Biopsy of the mediastinal mass reveals sheets of large cells with abundant cytoplasm, "starry-sky" appearance, high mitotic rate, and positive CD20, CD30, and PAX5 (weak) markers. Ki-67 proliferation index is 95%. What is the most likely diagnosis?