Bug: the autolink pass stashed <a> tags (via _al_stash) before running,
but did not stash <img> tags. When  was converted to an <img>
tag by the image pass, the subsequent autolink regex matched the URL
inside src="..." and wrapped it in <a href="...">url</a>, producing
src="<a href="...">url</a>" — a completely broken image source.
Fix: extend the _al_stash regex from:
(<a\b[^>]*>[\s\S]*?<\/a>)
to:
(<a\b[^>]*>[\s\S]*?<\/a>|<img\b[^>]*>)
This stashes both <a> and self-closing <img> tags before autolink runs,
then restores them after, so the URL inside src= is never touched.
Adds 7 regression tests in tests/test_issue487b.py.
2810233af4