--rollback: undo a rip-up that leaves fewer connections routed

The connections a rip-up route crosses are rerouted at once in free
space. If that leaves fewer connections routed than before, the new
route comes out and every displaced connection goes back on its old
path, with its rip count as it was. Even trades are kept.

src/freeroute/route/exact_router.py

@@ -98,6 +98,7 @@ def route_board_exact(
     seed: int | None = None,
     route_first: frozenset | None = None,
     access_cost: float = 0.0,
+    rollback: bool = False,
 ) -> ExactRouteResult:
     """Route ``board`` orthogonally and verify the result with exact geometry.
 
@@ -129,6 +130,7 @@ def route_board_exact(
         seed=seed,
         route_first=route_first,
         access_cost=access_cost,
+        rollback=rollback,
     )
     edge = 0 if edge_clearance is None else edge_clearance
     outline = board.outline