Patch updates to rendered HTML

[CaptainCodeman]

If the markdown renders images, videos or iframes in the HTML you can get some quite noticeable re-loading of content as you make edits:

Screen.Recording.2025-05-15.at.12.20.59.PM.mov

Using a HTML diff / patch lib can avoid some unnecessary updates:

Screen.Recording.2025-05-15.at.12.20.09.PM.mov

I'll create a PR ...