Function extract_from_markdown 

fn extract_from_markdown<T>(
    markdown: &[MarkdownDoc],
    kind: FenceKind,
    parser: impl Fn(Option<&str>, &str) -> Result<T, ConfigError>,
) -> Result<Vec<T>, PackError>

Generic helper that extracts items from Markdown fence blocks.

Scans all documents, users, and blocks for blocks matching kind, then calls parser on each match. This eliminates the repeated triple-nested loop that was duplicated across the six typed extraction functions.