--promote-failures: reroute with the connections that failed moved first

Each round routes every connection that has failed so far ahead of the
rest, which keep their order. The promoted set only grows, so two groups
cannot take turns failing; the best round is kept.

tests/route/test_orders.py

@@ -5,7 +5,13 @@ from __future__ import annotations
 from pathlib import Path
 
 from freeroute.cli import main
-from freeroute.route.orders import best_of_orders, candidate_seeds, route_once, score
+from freeroute.route.orders import (
+    best_of_orders,
+    candidate_seeds,
+    promote_failures,
+    route_once,
+    score,
+)
 
 FIXTURES = Path(__file__).resolve().parent.parent / "dsn" / "fixtures"
 # Greedy (no rip-up), the default order strands a net here and seeds 1 to 3 do not.
@@ -42,3 +48,31 @@ def test_cli_orders_writes_the_best_session(tmp_path, capsys):
     assert "tried 3 orders, kept --seed 1" in err
     assert "not routed" not in err
     assert out.read_text().startswith("(session")
+
+
+def test_promoting_the_failed_connection_routes_it():
+    best, rounds = promote_failures(RIPUP, "grid", GREEDY, rounds=3)
+    assert rounds == [(1, 0), (0, 0)]  # stops once nothing fails
+    assert score(best) == (0, 0)
+
+
+def test_nothing_promoted_is_the_plain_order():
+    plain = route_once(RIPUP, "grid", {}, None)
+    empty = route_once(RIPUP, "grid", {"route_first": frozenset()}, None)
+    assert plain == empty
+
+
+def test_cli_promote_failures(tmp_path, capsys):
+    out = tmp_path / "p.ses"
+    rc = main(
+        [
+            str(FIXTURES / "ripup_needed.dsn"),
+            "--no-rip-up",
+            "--promote-failures",
+            "3",
+            "-do",
+            str(out),
+        ]
+    )
+    assert rc == 0
+    assert "2 rounds, unrouted 1, 0" in capsys.readouterr().err

tests/test_cli.py

@@ -156,6 +156,7 @@ def test_cli_supported_combinations_are_accepted(tmp_path):
         "max_passes": ["-mp", "4"],
         "seed": ["--seed", "3"],
         "orders": ["--orders", "2"],
+        "promote": ["--promote-failures", "2"],
     }
     for engine, options in ENGINE_OPTIONS.items():
         for option in options: