With CR-only line endings, ExtractBody/RemoveFrontmatter/ReplaceFrontmatter keep the blank lines after the header ("\r\rBody" instead of "Body") #184

What's wrong

TrimBody (Frontmatter/Frontmatter.cs:502-519) only advances past leading blank lines on '\n'. SplitLines (Frontmatter.cs:465) documents support for CRLF, LF and CR, so CR-only documents parse fine but their leading blank body lines are never trimmed.

Repro (main @ 23c87b1)

Frontmatter.ExtractBody("---\rtitle: x\r---\r\r\rBody\r");        // "\r\rBody"   (LF equivalent gives "Body")
Frontmatter.RemoveFrontmatter("---\rtitle: x\r---\r\r\rBody\r");  // "\r\rBody" + NewLine
Frontmatter.ReplaceFrontmatter(...)                                // writes "---\ntitle: y\n---\n\r\rBody"

Expected: "Body", the same as for LF and CRLF input.

Why it matters

The output depends on line-ending style, and callers get stray blank lines at the start of the body. Low impact since CR-only files are rare, but it contradicts the line-ending support SplitLines advertises.

Suggested fix

In TrimBody, treat both terminators as line ends: if (c is '\n' or '\r') start = i + 1;. For CRLF this still lands after the \n.

Acceptance: ExtractBody/RemoveFrontmatter return the same body for CR, LF and CRLF versions of a document with blank lines after the header.