Complex order form presented as series of linked screens
It was a lack of useful earlier search results that led to posting the question. But often the query just needs the right search term. In this case, it's exactly "multistep" or "multi-step." That finds this, which along with the ensuing discussion and comments, answers the question quite well: 
https://www.drupal.org/node/324779 

So, the capability was already there in Webforms, I just hadn't seen how to use it.