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.

tests/route/test_fixed_wiring.py

@@ -0,0 +1,50 @@
+"""Tracks and vias already on the board (the DSN's wiring) are fixed copper."""
+
+from __future__ import annotations
+
+from freeroute.board import build_board
+from freeroute.dsn import parse_dsn
+from freeroute.route.pipeline import route_dsn_board_exact
+
+_DSN = """(pcb wired (parser) (resolution um 10) (unit um)
+  (structure
+    (layer F.Cu (type signal)) (layer B.Cu (type signal))
+    (boundary (path pcb 0  0 0  30000 0  30000 -20000  0 -20000  0 0))
+    (via "Via[0-1]_600:300_um") (rule (width 250) (clearance 200)))
+  (placement (component S (place A1 5000 -10000 front 0) (place A2 25000 -10000 front 0)))
+  (library
+    (image S (pin smd 1 0 0))
+    (padstack smd (shape (rect F.Cu -400 -400 400 400)) (attach off))
+    (padstack "Via[0-1]_600:300_um" (shape (circle F.Cu 600)) (shape (circle B.Cu 600)) (attach off)))
+  (network (net SIG (pins A1-1 A2-1)) (net GND))
+  (wiring
+    (wire (path F.Cu 400  15000 0  15000 -20000) (net GND) (type route))
+    (via "Via[0-1]_600:300_um"  8000 -14000 (net GND) (type route))))"""
+
+
+def test_wiring_is_read_with_its_nets():
+    dsn = parse_dsn(_DSN)
+    assert [(w.net, w.layer, w.width) for w in dsn.wires] == [("GND", "F.Cu", 400.0)]
+    assert [(v.net, v.x, v.y) for v in dsn.vias] == [("GND", 8000.0, -14000.0)]
+
+
+def test_wiring_is_fixed_copper_and_not_a_connection_to_route():
+    board = build_board(parse_dsn(_DSN))
+    copper = board.get_fixed_copper()
+    gnd = next(n.net_number for n in board.nets if n.name == "GND")
+    assert copper and all(net == gnd for net, _, _ in copper)
+    assert not any(gnd in p.net_nos for p in board.get_pins())
+
+
+def test_other_nets_route_around_a_fixed_track():
+    dsn = parse_dsn(_DSN)
+    exact, scale, names = route_dsn_board_exact(dsn, resolution=2)
+    sig = next(i + 1 for i, n in enumerate(dsn.nets) if n.name == "SIG")
+    assert sig in exact.result.routed_net_numbers
+    assert exact.drc_clean
+    wall = 15000 * scale
+    for layer, pts in exact.result.wires.get(sig, []):
+        if names[layer] != "F.Cu":
+            continue
+        for p, q in zip(pts, pts[1:], strict=False):
+            assert not (min(p.x, q.x) < wall < max(p.x, q.x)), "crossed the GND track on F.Cu"