Recognise frontmatter delimiters only as whole lines at the top of a document

[matt-edmondson]

Fixes #132
Fixes #133

The triage on #133 asked for both fixes in one PR, because they share a root cause.

What was wrong

ExtractFrontmatterObjects split the whole document on every --- followed by a newline:

• Any "---" in the body is treated as a frontmatter delimiter: horizontal rules get merged into the header and CombineFrontmatter duplicates later blocks #132: Horizontal rules in the body were read as delimiters, so a key: value paragraph between two rules was merged into the header. A value like foo--- split the header mid-line. CombineFrontmatter merged the second block into the header and also left it in the body.
• Frontmatter-only document without a trailing newline isn't parsed, and AddFrontmatter then discards its existing properties #133: A closing --- at the very end of the file wasn't recognised, so ExtractFrontmatter returned null. AddFrontmatter then treated the existing properties as empty and overwrote them.

Change

• New TrySplitFrontmatterBlocks:
  • Line-based parsing. The first line opens a block, and the block closes at the next line that is exactly ---; trailing whitespace is allowed.
  • A closing delimiter at the end of input counts.
  • Further blocks are consumed only while they follow directly. The body is everything after the last block consumed.
  • Lines split on CRLF, LF and CR wherever they occur, so the existing mixed-line-ending test still passes.
  • Blocks and body are sliced from the original text, so the body keeps its bytes exactly.
• AddFrontmatter guard, as the triage asked: when frontmatter is present but can't be read, the document comes back unchanged instead of being overwritten. An empty block (---\n---) still gets the new properties.
• An unclosed opening --- is now treated as no frontmatter. Before, everything after it was parsed as a block.

Tests

New DelimiterLineTests, 8 cases, covering each acceptance item:

• Horizontal rules around note: hi stay in the body, for both ExtractBody and CombineFrontmatter (exact output).
• A foo--- value mid-line.
• The two-block combine case, with an exact expected output and no duplicated author: B.
• Frontmatter-only input with no trailing newline, for both ExtractFrontmatter and AddFrontmatter (keeps title and author).
• An unreadable block is left untouched, and an empty block still gets new properties.

Against the original Frontmatter.cs, 6 of the 8 new tests fail. The other two cover behaviour that already worked and must not regress. With the fix, all 153 tests pass.

🤖 Generated with Claude Code

https://claude.ai/code/session_01VpbpayL5Kw99JfbzgkJfKa

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Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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