A 35-year-old man is brought to the emergency department 2 hours after a motor vehicle collision. He was unconscious at the scene for approximately 30 seconds, then regained consciousness and was alert during transport. Now he complains of severe headache and is becoming increasingly drowsy. On examination, his left pupil is dilated and sluggish. CT head (non-contrast) shows a lens-shaped hyperdense collection in the left frontoparietal region, tightly bounded by the coronal and sagittal sutures, with no midline crossing. What is the most likely diagnosis?