Function process_markdown_with_toc_to_writer 

pub fn process_markdown_with_toc_to_writer<W: Write>(
    content: &str,
    writer: &mut W,
    options: &MarkdownOptions<'_>,
) -> Result<Vec<Heading>, MarkdownError>

Streams processed HTML to writer and returns the table of contents collected during the AST walk.

Same shape as process_markdown_to_writer but with the toc metadata returned alongside the IO result.