Function collect_headings 

pub fn collect_headings<'a>(
    root: Node<'a>,
    prefix: Option<&str>,
) -> Vec<Heading>

Walks the AST and returns one Heading per heading node, in document order.

prefix mirrors MarkdownOptions::header_ids:

• None or Some("") → bare slug ("introduction").
• Some(p) → format!("{p}{slug}") ("user-content-introduction").

Uses comrak’s own Anchorizer so dedup behaviour (-1, -2 suffixes for repeated headings) matches the rendered HTML.