Function extract_toc 

pub fn extract_toc(doc: &Document) -> Vec<TocEntry>

Extract all top-level headings from a document as a flat list of TocEntry items.

• Explicit {#id} from the AST id field is used as-is; otherwise the slug is derived from the heading text via heading_slug.
• Duplicate slugs receive -1, -2, … suffixes in document order.

Slug parity with the renderer: the renderer increments its own slug counter for every heading it encounters during the depth-first tree walk — including headings that live inside blockquotes, admonitions, etc. This function replicates that walk so that the counters stay in sync. Nested headings are not listed in the output (callers expect a “structural” TOC), but their slugs are still counted so that the numbers assigned to top-level headings match the id attributes in the rendered HTML.