NixOS rollback: restore source tree to match the target generation

When a user asks to roll back a broken NixOS change, nixos-rebuild switch --rollback only flips the running system; the flake working tree still holds the broken config, so the next rebuild re-breaks it. Find the commit matching the good generation (repos that auto-commit per generation make this easy), git checkout <commit> -- <paths> for only the files that commit changed, and stash rather than discard uncommitted edits. Build the toplevel with nix build .#nixosConfigurations.<host>.config.system.build.toplevel --no-link to confirm before the user runs the sudo rollback. A sudden UI size change after a compositor config refactor is often a dropped output scale block, not the compositor itself.