A 68-year-old man with hypertension and on aspirin for secondary stroke prevention presents to the emergency department with acute onset severe headache, confusion, and right-sided weakness. His wife reports he fell from standing height 3 days ago and hit his head on the corner of a table, but seemed fine immediately after. On examination, he is drowsy, with a right pupil dilated to 5 mm, and right hemiparesis. CT head (non-contrast) shows a crescent-shaped hyperdense collection that does not cross the midline, with mass effect and midline shift. What is the most likely diagnosis?