--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.

src/freeroute/route/exact_router.py

@@ -95,6 +95,7 @@ def route_board_exact(
     resolution: int = 1,
     heuristic_weight: float = 1.0,
     seed: int | None = None,
+    route_first: frozenset | None = None,
 ) -> ExactRouteResult:
     """Route ``board`` orthogonally and verify the result with exact geometry.
 
@@ -124,6 +125,7 @@ def route_board_exact(
         resolution=resolution,
         heuristic_weight=heuristic_weight,
         seed=seed,
+        route_first=route_first,
     )
     edge = 0 if edge_clearance is None else edge_clearance
     outline = board.outline