qmd writer turns a loose bullet list into a tight one when items contain a nested sublist

A BulletList parsed as loose (each item's first block is Paragraph) round-trips as tight (Plain) when at least one item contains a nested sublist. The writer emits blank-line separators between top-level items but the re-parser does not treat the resulting source as loose, so the outer list collapses. Symptom: items[[i]][[1]]$tag = "Plain" after the round trip where the original was "Paragraph".

$ printf -- '-   a\n\n    -   c\n    -   d\n\n-   b\n' 
-   a

    -   c
    -   d

-   b

$ printf -- '-   a\n\n    -   c\n    -   d\n\n-   b\n' | cargo run --bin pampa --
[ BulletList [[Para [Str "a"], BulletList [[Plain [Str "c"]], [Plain [Str "d"]]]], [Para [Str "b"]]] ]

$ printf -- '-   a\n\n    -   c\n    -   d\n\n-   b\n' | cargo run --bin pampa -- -t qmd
* a
  
  * c
  * d

* b

$ printf -- '-   a\n\n    -   c\n    -   d\n\n-   b\n' | cargo run --bin pampa -- -t qmd | cargo run --bin pampa --
[ BulletList [[Plain [Str "a"], BulletList [[Plain [Str "c"]], [Plain [Str "d"]]]], [Plain [Str "b"]]] ]

Examples in quarto-web:

• https://github.com/quarto-dev/quarto-web/blob/8b1be24df1c9301f2700c77e41e7a9f789535717/docs/blog/posts/2024-04-01-manuscripts-rmedicine/index.qmd#L19
• https://github.com/quarto-dev/quarto-web/blob/8b1be24df1c9301f2700c77e41e7a9f789535717/docs/authoring/_shortcodes.qmd#L30
• https://github.com/quarto-dev/quarto-web/blob/8b1be24df1c9301f2700c77e41e7a9f789535717/docs/journals/authors.qmd#L142

Reproduce the round trip with the cargo run --bin pampa -- -t qmd entry point and compare the original and re-parsed BulletList structure. The fix is done when a loose outer list containing a nested sublist remains Paragraph-based after qmd writing and parsing, matching the issue's symptom.