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/grid_router.py

@@ -250,6 +250,7 @@ class GridRouter:
         # trace halos: node -> indices of the connections claiming it
         self._halo: dict[Node, list[int]] = {}
         self._rasterize_obstacles()
+        self._rasterize_fixed_copper()
 
     # --- grid helpers -------------------------------------------------------
 
@@ -360,6 +361,16 @@ class GridRouter:
                         if item.blocks_vias:
                             self._via_keepout.add((*cell, item.layer))
 
+    def _rasterize_fixed_copper(self) -> None:
+        """Tracks and vias already on the board: copper of their net, which
+        other nets keep the same distance from as from a pad."""
+        margin = self.clearance + self.half_width
+        for net, layers, box in self.board.get_fixed_copper():
+            on = [layer for layer in layers if layer in self.layers]
+            for cell in self._cells_in_box(box, margin):
+                for layer in on:
+                    self._pad_block.setdefault((*cell, layer), set()).add(net)
+
     # --- occupancy cost -----------------------------------------------------
 
     def _enter_cost(