Function existing_memory 

pub fn existing_memory(
    db: &Db,
    write_scope: Scope,
    content: &str,
) -> Result<Option<NodeId>, TopoError>

The id of a Memory in write_scope whose normalized content equals content. Hash-bucket lookup, then exact normalized-content verify on every candidate; oldest id wins. Superseded memories (those with a superseded_at timestamp) are excluded — re-learning a retired fact is a NEW fact, and the tombstone’s as_of history stays intact.