Having issues getting this to kick back the Table of Contents... but I pulled it from PhrictionContent. What's the best place to get the ToC as a separate entity?

The $engine you're using here is empty, while the $engine in the original is one that has been used to process the text.

If you do a purely standalone generation of the TOC, you'll have to do the work for generating it twice -- once to generate the body, then once to regenerate it to get the TOC.

Easiest fix for now at least is probably making didMarkupText() just have a side effect like:

$this->renderedTableOfContents = ...

...then add a method like:

public function getRenderedTableOfContents() {
  if ($this->renderedTableOfContents === null) {
    throw new PhutiIInalidStateException('didMarkupText');
  }
  return $this-> renderedTableOfContents;
}

...to fetch it. I think that will work until I can clean up Remarkup more generally.