Function render_section 

pub fn render_section(region: &str, new: &[(&Comment, Option<&str>)]) -> String

Render the full ** Active Comments subtree: the heading, the existing body preserved verbatim (A3), then each new comment whose id is not already present appended as a TODO heading.

Each new entry pairs a Comment with the CUSTOM_ID it anchors to (or None for document-level).