A --- rule on the first body line is read as a second frontmatter block: body text is deleted by ExtractBody/RemoveFrontmatter or merged into the header #140

What's wrong

TrySplitFrontmatterBlocks (Frontmatter/Frontmatter.cs ~371-396) loops while (IsDelimiterAt(next)). So after the header closes, a --- on the very next line opens another block, and that block runs to the next --- line anywhere in the document. The fix for #132 stopped horizontal rules further down the body being treated as delimiters, but a rule placed directly under the header is still consumed.

Repro

This was reproduced against current main (13a0d6f) using scratch MSTest methods.

Case A: the text between the rules isn't a YAML mapping, so it is lost.

---
title: A
---
---
Important paragraph

---
More

• ExtractBody returns "More".
• RemoveFrontmatter returns "More\n".
• ReplaceFrontmatter, AddFrontmatter and CombineFrontmatter all emit the header followed by only More.
• "Important paragraph" and both rules are silently deleted.
• Expected body: ---\nImportant paragraph\n\n---\nMore.

Case B: the text happens to parse as YAML, so it moves into the header.

---
title: My Post
---
---
Note: read this first

---

# Heading

CombineFrontmatter returns ---\ntitle: My Post\nNote: read this first\n---\n\n# Heading\n. A body paragraph has become a header property.

For comparison, ---\ntitle: A\n---\n---\n\nIntro text\n, which has no second rule, keeps its body correctly.

Why it matters

Placing a thematic break directly under the header is common Markdown. Running any of the rewriting APIs over such a document destroys or relocates content with no error. This isn't #139: that issue is about CombineFrontmatter dropping an unparseable block, whereas this bug is in the splitter and affects every API, including ExtractBody and RemoveFrontmatter.

Suggested fix / acceptance criteria

• Accept a block after the first only if its content parses as a non-empty YAML mapping. Otherwise stop splitting and leave the --- in the body. This fixes Case A.
• Case B is ambiguous, because consecutive blocks are supported on purpose (DelimiterLineTests.cs:55). Decide on a tighter rule, for example rejecting a follow-on block whose content is surrounded by blank lines, and add a regression test for it.
• Add a test for Case A in which ExtractBody returns the full body, including both rules.