--seed: shuffle the order connections are routed in

The router is deterministic, but its result depends heavily on which
connection routes first. A seed shuffles that order reproducibly, so a
change can be judged over several orders instead of the one it happened
to get.

tests/test_cli.py

@@ -154,6 +154,7 @@ def test_cli_supported_combinations_are_accepted(tmp_path):
         "diagonal": ["--diagonal"],
         "rip_up": ["--no-rip-up"],
         "max_passes": ["-mp", "4"],
+        "seed": ["--seed", "3"],
     }
     for engine, options in ENGINE_OPTIONS.items():
         for option in options: