Function build 

pub fn build(
    label: &str,
    texts: &[(String, String)],
    filter: Option<GlobSet>,
    keys: Option<Vec<String>>,
) -> Graph

Build the frontmatter graph fragment from text files, labeled label. filter scopes which paths the builder reads (None reads all); keys scopes which frontmatter keys yield edges (None uses shape detection). The fragment carries edges plus a node per file whose frontmatter parses to an object.