Rollback on by default; --no-rollback keeps every rip-up

Over 8 net orders it left 25 to 30 fewer connections unrouted on the
dense sense boards and 16 to 18 fewer on two PCBench boards, and was
never clearly worse. KiCad DRC on the sense harness: 41 opens to 28.

src/freeroute/cli.py

@@ -46,7 +46,7 @@ _FLAG_NAMES = {
     "orders": "--orders",
     "promote": "--promote-failures",
     "access": "--access-cost",
-    "rollback": "--rollback",
+    "rollback": "--no-rollback",
 }
 
 
@@ -168,10 +168,12 @@ def build_parser() -> argparse.ArgumentParser:
         "another pin's way out while that pin still has connections to route (default 0: off)",
     )
     parser.add_argument(
-        "--rollback",
-        action="store_true",
-        help="grid and exact engines: reroute the connections a rip-up displaces at once, "
-        "and undo the rip-up if fewer connections end up routed",
+        "--no-rollback",
+        dest="rollback",
+        action="store_false",
+        help="grid and exact engines: keep every rip-up, as before. By default the "
+        "connections a rip-up displaces are rerouted at once and the rip-up is undone "
+        "if fewer connections end up routed",
     )
     parser.add_argument(
         "--jobs",
@@ -202,7 +204,7 @@ def main(argv: list[str] | None = None) -> int:
             ("orders", args.orders is not None),
             ("promote", args.promote_failures is not None),
             ("access", args.access_cost is not None),
-            ("rollback", args.rollback),
+            ("rollback", not args.rollback),
         )
         if asked
     ]

src/freeroute/route/exact_router.py

@@ -98,7 +98,7 @@ def route_board_exact(
     seed: int | None = None,
     route_first: frozenset | None = None,
     access_cost: float = 0.0,
-    rollback: bool = False,
+    rollback: bool = True,
 ) -> ExactRouteResult:
     """Route ``board`` orthogonally and verify the result with exact geometry.
 

src/freeroute/route/grid_router.py

@@ -162,7 +162,7 @@ class GridRouter:
         seed: int | None = None,
         route_first: frozenset | None = None,
         access_cost: float = 0.0,
-        rollback: bool = False,
+        rollback: bool = True,
     ) -> None:
         self.board = board
         # Pin access: while a pin still has connections to route, the cells
@@ -174,7 +174,9 @@ class GridRouter:
         self._pending: dict[int, int] = {}
         # Rip-up as a transaction: the connections a rip-up route crosses are
         # rerouted at once, and if fewer connections are routed afterwards
-        # than before, everything goes back as it was.
+        # than before, everything goes back as it was. On by default: over
+        # 8 net orders it left 25 to 30 fewer connections unrouted on dense
+        # boards and was never clearly worse.
         self.rollback = bool(rollback)
         # None keeps the deterministic order; a seed shuffles which connection
         # routes first, to measure how much a result owes to the order
@@ -1228,7 +1230,7 @@ def route_board(
     seed: int | None = None,
     route_first: frozenset | None = None,
     access_cost: float = 0.0,
-    rollback: bool = False,
+    rollback: bool = True,
 ) -> RouteResult:
     """Convenience wrapper: build a :class:`GridRouter` and route the board."""
     router = GridRouter(

src/freeroute/route/pipeline.py

@@ -83,7 +83,7 @@ def route_dsn_board(
     seed: int | None = None,
     route_first: frozenset | None = None,
     access_cost: float = 0.0,
-    rollback: bool = False,
+    rollback: bool = True,
 ) -> tuple[RouteResult, int, list[str]]:
     """Route a parsed :class:`DsnBoard`; return the board-unit result, the scale,
     and the layer names. ``layers`` restricts routing to those signal-layer
@@ -151,7 +151,7 @@ def build_routing_result(
     seed: int | None = None,
     route_first: frozenset | None = None,
     access_cost: float = 0.0,
-    rollback: bool = False,
+    rollback: bool = True,
 ) -> RoutingResult:
     """Route ``dsn`` (grid track) and convert to a DSN-unit RoutingResult."""
     route, scale, layer_names = route_dsn_board(
@@ -178,7 +178,7 @@ def route_dsn_board_exact(
     seed: int | None = None,
     route_first: frozenset | None = None,
     access_cost: float = 0.0,
-    rollback: bool = False,
+    rollback: bool = True,
 ) -> tuple[ExactRouteResult, int, list[str]]:
     """Route a parsed :class:`DsnBoard` with the exact-geometry (orthogonal,
     DRC-verified) track; return the exact result, the scale, and layer names.
@@ -225,7 +225,7 @@ def build_exact_routing_result(
     seed: int | None = None,
     route_first: frozenset | None = None,
     access_cost: float = 0.0,
-    rollback: bool = False,
+    rollback: bool = True,
 ) -> RoutingResult:
     """Route ``dsn`` (exact track) and convert to a DSN-unit RoutingResult."""
     exact, scale, layer_names = route_dsn_board_exact(

tests/route/test_rollback.py

@@ -38,6 +38,6 @@ def test_occupancy_only_holds_routed_connections(name):
 @pytest.mark.parametrize("name", ["ripup_needed.dsn", "true_density.dsn", "kicad_routable.dsn"])
 def test_rollback_never_routes_fewer_than_plain_rip_up(name):
     text = (FIXTURES / name).read_text()
-    plain, _, _ = route_dsn_board(parse_dsn(text))
-    undo, _, _ = route_dsn_board(parse_dsn(text), rollback=True)
+    plain, _, _ = route_dsn_board(parse_dsn(text), rollback=False)
+    undo, _, _ = route_dsn_board(parse_dsn(text))
     assert undo.unrouted <= plain.unrouted

tests/test_cli.py

@@ -158,7 +158,7 @@ def test_cli_supported_combinations_are_accepted(tmp_path):
         "orders": ["--orders", "2"],
         "promote": ["--promote-failures", "2"],
         "access": ["--access-cost", "5"],
-        "rollback": ["--rollback"],
+        "rollback": ["--no-rollback"],
     }
     for engine, options in ENGINE_OPTIONS.items():
         for option in options: