What has shut off rendering and display of the world background? Yesterday I was playing with world settings while testing lighting, and now I can't get it to display again as the world background in the 3d view. I've gone over every setting I can think of - the viewport display setting have world background checked, all the ray visibility checkboxes are ticked for the world settings, the nodes for the world are all set the way they were before, and 'Use Nodes' is checked. I even tried rebuilding the world's nodes, and the new world doesn't display either. That makes me suspicious the issue is one of those many checkboxes somewhere being ticked or unticked, but after hunting for a while, I can't find anything.

It shows up in the Preview box - it's subtle, because it is a starry sky, but it's there. I'm stuck. What have I missed? I'm just putting an image of every relevant setting I can think of here.
Switch to perspective view mode in the viewport (Numpad 5). When in orthographic view, which is what you are in now, the world background cannot be properly displayed. Instead, you get a single color based on (I think) the color of the pixel that you are facing. If you have a background with more colors, you will see the color change as you rotate around. Since your background is almost all black, you likely won't see any color change as you rotate your view.