Wood – How could one create a man sized cylinder be created with woodworking

The right way to do this is to deconstruct the side and top of the stairway wall - remove the drywall, the stair railing, and the top plate for that wall, and then add additional studs.

Your two choices are balloon framing that runs all the way to the ceiling and platform framing that is two separate walls stacked on top of each other. For platform framing, you would need to tie the top/bottom plates to the left and right walls so it would not move. Balloon framing would avoid this, so that would be my choice. Make sure to put proper fire blocking in. The stairs may be hung on the current wall, so you will want to make sure you don't mess up the current wall's structure. You can do this by sistering the new studs next to the old ones.

This is a pretty straightforward thing to do.

I would not recommend adding a full wall next to the existing wall; you would have to modify the floor covering and extend any utilities out to the surface of the new wall.

I hate to have to rain on your parade but there is a reason you do not find ready made wooden curtain rods as you describe. At a span of 76 inches even the straight part of a dowel rod that is less than an inch in diameter will sag over time. Add to that the extra weight of black out curtains and the straight part of the rod will be even less effective. This is the reason that the usual straight wood curtain rods that you find will be at least 1.5 inches in diameter if not even 2 inches. And at your span such rods still need to be supported in the middle which leads to ring hung curtains having to be center opening toward two sides.

You have the additional consideration that ring hung curtains on an open rod leave considerable space at the top for light to come in negating the effectiveness of the black out considerations and your stated objective to want to have the curtains seal off light at the side returns to the wall. Top sealing for light shielding can be done with a box type valance structure this hides the curtain rod assembly so there is little point to try to design the curtain support to the aesthetic of the ring hung style as you pictured in your question. And if you try to avoid the look of the box and use a double rod structure with the outer one hanging a valance it has to fit tight up to the ceiling and that would hide the other rod the curtain hangs from....so it just as well may be a ready made metal one even avoiding the the ring hung style.