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/dsn/model.py

@@ -27,6 +27,8 @@ __all__ = [
     "NetPin",
     "Net",
     "NetClass",
+    "WiringVia",
+    "WiringWire",
     "Keepout",
     "DsnBoard",
 ]
@@ -186,6 +188,26 @@ class Keepout:
     kind: str = "keepout"  # keepout | via_keepout | wire_keepout | place_keepout
 
 
+@dataclass
+class WiringWire:
+    """A track already on the board: ``(wire (path LAYER WIDTH x y ...) (net N))``."""
+
+    net: str
+    layer: str
+    width: float
+    coords: list[float] = field(default_factory=list)
+
+
+@dataclass
+class WiringVia:
+    """A via already on the board: ``(via PADSTACK x y (net N))``."""
+
+    net: str
+    padstack: str
+    x: float
+    y: float
+
+
 @dataclass
 class DsnBoard:
     """The fully parsed DSN file."""
@@ -197,6 +219,9 @@ class DsnBoard:
     layers: list[Layer] = field(default_factory=list)
     #: copper zones, as planes of a net
     planes: list[Plane] = field(default_factory=list)
+    #: copper already routed (the ``wiring`` scope), kept fixed
+    wires: list[WiringWire] = field(default_factory=list)
+    vias: list[WiringVia] = field(default_factory=list)
     #: bounding-box boundary shape (layer ``pcb``), if present
     boundary: Shape | None = None
     #: outline shapes (layer ``signal``)