Function serialize_whatsout 

pub fn serialize_whatsout(doc: &PostDocument, mode: Whatsout) -> String

Apply the requested Whatsout extraction to doc and return the serialized subtree. Returns the full document for Whatsout::Document (the default no-op) or when extraction finds no candidate node.

Wraps the two get_* helpers + libxml node_to_string so callers don’t have to thread the inner [libxml::tree::Document] through.