Pandoc-style ... closing marker is not recognised, so the frontmatter runs on to a later --- rule and body text is deleted #152

What's wrong

YAML (and Pandoc's metadata blocks) allow a YAML block opened by --- to be closed by the end-of-document marker .... IsDelimiterLine (Frontmatter/Frontmatter.cs, ~line 436) only accepts ---, and the closing scan in TrySplitFrontmatterBlocks (~line 375) uses it. So a block closed with ... is not closed; the scan keeps going until the next --- anywhere in the document, typically a markdown horizontal rule.

Repro

string doc = "---\ntitle: A\n...\n# Heading\n\nImportant paragraph.\n\n---\n\nMore text\n";

Frontmatter.RemoveFrontmatter(doc);
// "More text\n"            <- "# Heading" and "Important paragraph." are gone

Frontmatter.ReplaceFrontmatter(doc, new() { { "title", "B" } });
// "---\ntitle: B\n---\nMore text\n"   <- same body loss

Frontmatter.ExtractBody(doc);        // "More text"
Frontmatter.ExtractFrontmatter(doc); // null, although "title: A" is valid frontmatter

With no later --- the document is treated as having no frontmatter at all, e.g. AddFrontmatter returns the input unchanged rather than recognising the existing title: A block.

Why it matters

Documents authored for Pandoc commonly use ... as the closing marker. Running Remove/Replace/ExtractBody on them silently deletes body content up to the first horizontal rule. This is data loss on write, not just a missed read.

This differs from #140 (a --- rule on the first body line) and #139 (Combine deleting an unparseable block): here the block boundary itself is wrong.

Suggested fix

Accept a line whose TrimEnd() is ... as a closing delimiter only (never as an opener) in the close loop of TrySplitFrontmatterBlocks. Keep the original closer when rewriting, or normalise to ---; either is fine as long as it is documented.

Acceptance criteria

• ExtractFrontmatter on the document above returns { title: A }.
• RemoveFrontmatter/ExtractBody return a body that starts with # Heading.
• A lone ... line at the top of a document is still not treated as an opener.