Rendering to CommonMark with a column width can create a heading

I see you've carefully avoided accidentally starting a list:

$ printf 'abcd x)' | cmark -t commonmark --width 4
abcd
x)
$ printf 'abcd 1)' | cmark -t commonmark --width 4
abcd 1)

But a heading can be accidentally created:

$ printf 'abcd =' | cmark -t commonmark --width 4 | cmark -t html
<h1>abcd</h1>
$ printf 'abcd -' | cmark -t commonmark --width 4 | cmark -t html
<h2>abcd</h2>

I think you can find this bug (and perhaps others) by writing a program that converts to HTML both directly and via CommonMark with width 4 and compares the two results and aborts if they're different, then fuzzing that program with AFL.

Start with the cmark command and reproduce the two width-4 examples that turn trailing '=' or '-' into headings when converted through CommonMark. Compare direct HTML output with the width-4 CommonMark round trip, then use the suggested AFL fuzzing approach; done means the outputs agree without accidental headings, including the reported cases.