A 68-year-old man with chronic alcoholism presents to the emergency department with a 3-week history of progressive confusion, gait disturbance, and left-sided weakness. His daughter reports he had a minor head injury from a fall 3 weeks ago, which he did not consider serious at the time. On examination, he is drowsy, has a left extensor plantar response, and mild left hemiparesis. CT head (non-contrast) shows a crescent-shaped collection that crosses the midline and compresses the lateral ventricles. What is the most likely diagnosis?