Read the DSN's wiring as fixed copper of its nets

Tracks and vias already on the board were ignored, so other nets could
be routed straight through them (the flight controller's 82 GND
stitching vias and its guard tracks). They are now copper of their net:
other nets keep a pad's distance from them in the grid, the exact check
and the room engine, and they add no connections to route.

src/freeroute/route/room_router.py

@@ -515,6 +515,10 @@ def route_board_rooms(
         pin_layers = frozenset(layer for layer in pin.layers if layer in signal_layers)
         if pin_layers:
             tree.insert(TreeShape(next(owner), net, pin_layers, box))
+    for net, copper_layers, box in board.get_fixed_copper():
+        on = frozenset(layer for layer in copper_layers if layer in signal_layers)
+        if on and not box.is_empty():
+            tree.insert(TreeShape(next(owner), net, on, box))
     for obstacle in board.get_obstacle_areas():
         if obstacle.layer not in signal_layers:
             continue