Hide YAML frontmatter in markdown preview

[OliverBennettdev]

Summary

• hide YAML frontmatter when it appears at the very start of a Markdown file
• preserve original source positions by masking frontmatter characters instead of deleting lines
• keep thematic breaks elsewhere in the document unchanged
• leave unterminated opening delimiters as ordinary Markdown
• add regression coverage for preview output and source-line mapping

Root cause

src/lib/markdown.ts passed the source directly to markdown-it. Without a frontmatter rule, the opening --- was rendered as a thematic break and the closing --- could turn the metadata above it into a setext heading.

Implementation

This uses a small dependency-free maskYamlFrontmatter helper. It replaces only non-newline characters in a leading, closed --- block with spaces before markdown parsing. That hides the metadata while preserving both line and character positions, so the existing data-sline / data-eline mapping remains aligned with the original source.

The masked source is also used for fenced-language discovery, so metadata cannot trigger unnecessary syntax-highlighter loading.

Validation

• helper compiles with TypeScript
• local behavior checks cover LF, CRLF, mid-document thematic breaks, unterminated frontmatter, and a closing delimiter at EOF
• regression tests added in tests/markdown.test.ts
• GitHub Actions will validate repository typechecking and Rust checks on this PR

Closes #134

[mattenarle10]

The implementation matches #134 and the regression coverage looks focused, including source-line preservation, CRLF handling, mid-document thematic breaks, and unterminated frontmatter. I do not see any GitHub CI checks reported for this branch yet, so please enable/run the repository checks before merge. Once CI is green, this can close #134.

[mattenarle10]

Thanks for the focused fix and regression tests. The frontmatter masking preserves source-line mapping and keeps regular thematic breaks working, which is exactly what the preview needed. Merged, and #134 is now closed.