Quick Update . . .
Going through some video I took a couple weeks ago, I noticed some odd behavior when the front tire hits a bump. If you look closely (make sure you're viewing at 1080p/60fps), you can see that the wheel oscillates in the fore-aft direction more than it does up and down after the impact.
This oscillation approximately lines up with what the clunk sounds like . . . the strut mount coming unseated from the tower and banging against it, either due to bolt stretch or tower deflection (see my post #10 in this thread).
Stiff or lowered suspension would make this problem worse . . . if the wheel can't move up to get out of the way of an impact, it'll have to move rearward. In other videos, I can tell that the noise doesn't happen until the suspension stops moving upwards and continues to "rattle" as it rebounds in the negative direction.
The dealer can't find anything and my buddy who works for Bosch calibrating this same steering gear on Corvette and V series says he's never seen a noise like this come from their gears.
This
has to be the problem. Now the question is . . . how to fix it?