Function project_edges

pub fn project_edges<M>(graph: &Graph, mapper: M) -> ProjectedGraph
where
    M: Fn(&Edge) -> Option<MappedEdge>,

Relabels raw graph edges and cumulates equal projected endpoint pairs.

Each returned ProjectedEdge owns the raw edges it represents, preserving concrete evidence for later violation messages. Results are ordered by projected (source_label, target_label).