Function render 

pub fn render(
    from: &str,
    body: &str,
    reference: Option<Reference<'_>>,
    attachments: &[String],
) -> String

Render one delivery: the source, the body, the material it quotes, and the absolute paths of its attachments.

Every block stands on its own: an absent body, reference, or attachment list renders no block at all rather than a labelled empty one. The body travels byte for byte — no trimming, no reflowing, no fence added around it — so an operator reading the model’s context reads the bytes the sender wrote.