--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/route/test_router.py

@@ -264,6 +264,19 @@ def test_larger_board_still_connects_its_multipin_nets():
     assert len(result.routed_net_numbers) == multi_pin == 4
 
 
+def test_a_seed_is_repeatable_and_does_change_the_order():
+    # The benchmark samples seeds to measure how much a result owes to the
+    # routing order. A seed that changed nothing would make every sample the
+    # same run and report no spread at all.
+    text = (FIXTURES / "kicad_routable.dsn").read_text()
+    default = _wire_key(route_dsn_board(parse_dsn(text))[0])
+    runs = {s: route_dsn_board(parse_dsn(text), seed=s)[0] for s in range(4)}
+    again = route_dsn_board(parse_dsn(text), seed=1)[0]
+    assert _wire_key(again) == _wire_key(runs[1])
+    assert any(_wire_key(r) != default for r in runs.values())
+    assert all(r.unrouted == 0 for r in runs.values())
+
+
 # --- oracle parity ----------------------------------------------------------