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/board/items.py

@@ -114,6 +114,9 @@ class Via(Item):
 
     padstack_name: str = ""
     location: IntPoint = field(default_factory=lambda: IntPoint(0, 0))
+    #: layer indices the via's pads occupy, and the largest pad as a box
+    layers: list[int] = field(default_factory=list)
+    shape: object = None
 
 
 @dataclass