Function diff 

pub fn diff(
    conn: &Connection,
    t1: &str,
    t2: &str,
    scope_path: Option<&str>,
) -> Result<GraphDiff>

Compare the graph state at two different timestamps.

• added — edges valid at t2 whose (from_id, to_id, relation) triple was not present at t1.
• removed — edges valid at t1 whose triple was not present at t2.
• changed — triples present at both t1 and t2 but with a different id (i.e. the edge was superseded), implying the properties or confidence changed.

When scope_path is Some(prefix), the diff is limited to edges within that scope hierarchy. When None, all scopes are compared (backward compatible).