Here’s their constitution any peace deal must be ratified by the people.

Which, I’m not going to say there isn’t going to be eventual concessions, those people aren’t going to vote to become a Russian state for a long time. Too much division. There are legitimate Russia supporters and there are a ton of pro-Ukrainian citizens. I don’t see how you do that, with actual democracy, for quite some time. Someone has to start losing fairly decisively sadly. I don’t see that happening for a while. A lot of people are going to die yet.

Edit: article 156 more specifically as changing borders requires a constitutional change which requires the people to pass.