Thank you, Andrew! The missing pieces in my "pixel peeping" were that I had been using ProPhoto RGB rather than Adobe RGB (duh!), and had not assigned my monitor profile. As soon as I did those steps, the banding disappeared to the point where the gradient seemed nice and smooth. At a couple of points I could vaguely detect a hint of banding, but I am not even sure whether I actually saw it, with my eyes about 2 inches from the monitor screen.
I also saw the first hint of separation when the lower point on the curve was at 3. I am not particularly shocked by that, as the NEC 3090 uses an H-IPS panel, which does not have the blackest of blacks as its native black point.
I did notice another artifact when viewing close up. While the steps between 3 and 255 were all neutral in appearance, I noticed when my eyes were close to the monitor a vague speckling of tiny magenta points. This seemed to be not a color cast per se but appeared like something sparkly from "behind" the screen. At my usual viewing distance I saw only neutral tones from black through grays to white. Could this be inherent in the H-IPS screen?