Function build_changeset 

pub fn build_changeset(
    db: &Db,
    peer_id: Option<&str>,
    name: &str,
) -> Result<Changeset>

Build this device’s export for peer_id — every row past the peer’s acked cursor per table (no cursor → full export, the first-run bootstrap), the un-acked tombstones, and the file manifest. Does not advance any cursor: that happens only when the peer acks.