Fix bug with multi-line list items

[dansteeves68]

I noticed the following unwanted behavior where an extra line is inserted into a multi-line list item.

This is my first time doing any rust. The test and solution are vibe coded with Opus 4.7.

Here is a shell session showing the bug:

$ cat -p tests/fixtures/multi-line-list-items.md                       
Here is a list
 
* this is a long-ish list item that needs to be longer than
  sixty chars followed by another one
* this is a list item

$ rust/target/release/md-fixup tests/fixtures/multi-line-list-items.md 
Here is a list

* this is a long-ish list item that needs to be longer than

  sixty chars followed by another one
* this is a list item

[ttscoff]

I'm going to merge this, but tests are currently failing after my last PR merge and I may need to do some fixes.