Tighter A* estimate on the orthogonal track, and --heuristic-weight

The grid search estimated the remaining distance as a straight line even
when only orthogonal steps exist; Manhattan is the tighter estimate those
moves admit and still never overestimates. --heuristic-weight W (default
1) makes it weighted A*: routes at most W times the shortest (tested).

src/freeroute/route/exact_router.py

@@ -93,6 +93,7 @@ def route_board_exact(
     via_half: int | None = None,
     edge_clearance: int | None = None,
     resolution: int = 1,
+    heuristic_weight: float = 1.0,
 ) -> ExactRouteResult:
     """Route ``board`` orthogonally and verify the result with exact geometry.
 
@@ -120,6 +121,7 @@ def route_board_exact(
         via_half=via_half,
         edge_clearance=edge_clearance,
         resolution=resolution,
+        heuristic_weight=heuristic_weight,
     )
     edge = 0 if edge_clearance is None else edge_clearance
     outline = board.outline