fix: Markdown.ToMd preserves YAML frontmatter

When Markdown.ToMd serialises a parsed MarkdownDocument back to Markdown
text, YamlFrontmatter paragraphs were silently dropped. Documents
containing a YAML front-matter block (--- ... ---) would lose that block
on round-trip.

Fix: emit the --- delimiters and frontmatter lines in formatParagraph so
round-trips are lossless.

Add two new tests: one with populated frontmatter and one with an empty
block.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

RELEASE_NOTES.md

@@ -2,6 +2,9 @@
 
 ## [Unreleased]
 
+### Fixed
+* Fix `Markdown.ToMd` silently dropping YAML frontmatter when serialising a parsed `MarkdownDocument` back to Markdown text. Frontmatter is now preserved with its `---` delimiters.
+
 ## [22.0.0] - 2026-04-03
 
 ### Fixed

src/FSharp.Formatting.Markdown/MarkdownUtils.fs

@@ -235,7 +235,14 @@ module internal MarkdownUtils =
               let lines = code.Replace("\r\n", "\n").Split('\n') |> Array.toList
               yield! lines
           //yield ""
-          | YamlFrontmatter _ -> ()
+          | YamlFrontmatter(lines, _) ->
+              yield "---"
+
+              for line in lines do
+                  yield line
+
+              yield "---"
+              yield ""
           | Span(body = body) -> yield formatSpans ctx body
           | QuotedBlock(paragraphs = paragraphs) ->
               for paragraph in paragraphs do

tests/FSharp.Markdown.Tests/Markdown.fs

@@ -1371,3 +1371,19 @@ let ``ToMd round-trip: indirect image with unresolved reference`` () =
     let result = Markdown.ToMd(doc)
     // When key is not resolved, should preserve the indirect form
     result |> should contain "![alt text][unknown-ref]"
+
+[<Test>]
+let ``ToMd preserves YAML frontmatter`` () =
+    let input = "---\ntitle: My Page\ndate: 2024-01-01\n---\n\nHello world.\n"
+    let result = input |> toMd
+    result |> should contain "---"
+    result |> should contain "title: My Page"
+    result |> should contain "date: 2024-01-01"
+    result |> should contain "Hello world."
+
+[<Test>]
+let ``ToMd preserves empty YAML frontmatter`` () =
+    let input = "---\n---\n\nHello.\n"
+    let result = input |> toMd
+    result |> should contain "---"
+    result |> should contain "Hello."