At full resolution, the line gets terrible.

Perhaps I've missed something, but it sounds like you want to make the line between the light and dark parts of the wall as sharp as possible, more like the lights on the right hand side of this image than the ones on the left:

In that case, as long as your camera is properly focussed in the first place, the real issue is not the camera but the lights. The sharpness of the top edge of the headlight beams is due to the geometry of the lights (size of light source, shape of reflector, lens arrangement, position of any baffles). Again, if I understand the question, you're really asking about headlights rather than photography. Check this reddit page for some information about projectors in headlights.

In your comment below, you asked about taking a photo where the dark portions of the image still have visible detail, as in the image above. To achieve that, you'll need to change the balance of light. There's not much you can do to change the brightness of the headlights, but you can make them seem less intense by using a faster shutter speed, or a smaller aperture, or by decreasing the camera's sensitivity (lower ISO setting). That will help bring out some of the detail in the light parts of the image rather than having those parts be completely washed out. Once you've got that setting, you'll need to add more light to the rest of the image to make details in the dark areas visible. You can do that by just turning on the lights in the garage. If you have a set of work lights handy, you could use them if you need more light. You can also use one or more speedlights if you have them, but the built-in flash that some cameras have might not be enough to compete with the headlights. You could also use daylight -- point the car at the back wall of the garage and open the garage door as much as necessary to provide the level of ambient light that you want.