Don't cost a pin stub for a route that ends on a plane

A plane end is marked with its zone, not a pin, so a route from a pad
outside the plane's outline crashed looking for the pin's location
(the flight-controller board's first placement).

src/freeroute/route/grid_router.py

@@ -451,10 +451,11 @@ class GridRouter:
         g_score: dict[Node, float] = {}
         came_from: dict[Node, Node] = {}
         # a stub costs its length, so the nearest cell wins unless it is blocked
-        goal_stub = {n: self._stub_cost(p, n) for n, p in goals.items() if p is not None}
+        # a pin end costs its stub; a zone or plane end is entered normally
+        goal_stub = {n: self._stub_cost(p, n) for n, p in goals.items() if isinstance(p, Pin)}
         for s in sorted(starts):
             pin = starts[s]
-            g_score[s] = self._stub_cost(pin, s) if pin is not None else 0.0
+            g_score[s] = self._stub_cost(pin, s) if isinstance(pin, Pin) else 0.0
             heapq.heappush(open_heap, (g_score[s] + self._h(s, goal_cell), s))
 
         # Via footprints are checked lazily: a layer change goes on the heap as