Function build_distributed_graph 

pub fn build_distributed_graph(
    edges: &[(usize, usize)],
    n_vertices: usize,
    config: &DistributedGraphConfig,
) -> DistributedGraph

Build a DistributedGraph from an edge list.

Steps:

1. Assign each vertex to a partition using the configured method.
2. Assign each edge to a shard:
   • EdgeCut / HashBased: edge goes to the source vertex’s shard.
   • VertexCut: edge goes to the shard of the endpoint with fewer edges.
   • Fennel: same as EdgeCut after FENNEL vertex assignment.
3. Record mirror vertices for cross-shard edges.
4. Build vertex_map for O(1) location queries.