Constant FOREIGN_LINEAGE_MARKER 

const FOREIGN_LINEAGE_MARKER: &[u8] = b"populate_userroomid_leftstate_table";

A marker written by a sibling conduwuit-lineage server but never by tuwunel. Its presence identifies a foreign database at a higher schema number even after tuwunel has stamped its own server_name, so a database opened by both servers in turn keeps booting rather than being refused as too new.