Add dragonfly congestion chord diagram visualization

                tuple(sorted(edge)): 0.0 for edge in self.net_graph.edges()

            }

    def dump_link_loads(self, path: str, *, dt: float | None = None) -> None:

        """Write current global_link_loads to a CSV file (src, dst, bytes).

        The resulting CSV can be fed directly to

        ``scripts/plot_dragonfly_congestion.py``.

        Parameters

        ----------

        path : str | Path

            Destination CSV file path.

        dt   : float, optional

            Simulation timestep (seconds).  If supplied it is written to a

            header comment so the plot script can auto-detect it.

        """

        import csv as _csv

        from pathlib import Path as _Path

        _Path(path).parent.mkdir(parents=True, exist_ok=True)

        with open(path, "w", newline="") as f:

            if dt is not None:

                f.write(f"# dt={dt}\n")

            writer = _csv.writer(f)

            writer.writerow(["src", "dst", "bytes"])

            for (u, v), b in self.global_link_loads.items():

                if b > 0:

                    writer.writerow([u, v, f"{b:.0f}"])

    def plot_topology(self, output_dir):

        """Plot network topology - save as png file in output_dir."""

        if output_dir: