Route with copper zones, and let vias into no-tracks areas

freeroute ignored the DSN's planes (KiCad's zones), so it never knew that
pads in a pour are joined by it. On the rc4 power stage those pads sit
inside "keep router off" areas as well, and every connection to them was
dropped without a word.

- Read (plane <net> <shape>) and put each zone on the board as a
  ConductionArea of its net. Zones are not obstacles: KiCad refills them
  around other nets' copper.
- The grid groups each net's pads and zone pieces that copper already
  joins (a pad in a zone, two zones sharing a pad) and routes only between
  groups. A connection may end on a zone by a via inside its copper, or by
  a track where tracks are allowed.
- wire_keepout now blocks traces only, as KiCad's "no tracks" rule area
  does; via_keepout only vias; keepout both, in the grid and in the exact
  verifier.

rc4 power stage, exact engine, resolution 2: 2 unconnected, down from 8
(FreeRouting JAR: 12), no DRC errors.

src/freeroute/dsn/reader.py

@@ -183,6 +183,11 @@ def _read_structure(node: SExp, board: DsnBoard) -> None:
             area = read_area(it)
             if area is not None:
                 board.keepouts.append(Keepout(area=area, kind="wire_keepout"))
+        elif head == "plane":
+            # (plane <net> <shape>): KiCad exports each zone this way
+            plane = _read_plane(it)
+            if plane is not None:
+                board.planes.append(plane)
         elif head == "place_keepout":
             area = read_area(it)
             if area is not None:
@@ -513,3 +518,19 @@ def parse_dsn(text: str) -> DsnBoard:
         # `wiring` and unknown scopes are intentionally ignored for now.
 
     return board
+
+
+def _read_plane(node: SExp):
+    """``(plane <net> (polygon <layer> <width> x y ...))`` -> :class:`Plane`."""
+    from .model import Plane
+
+    items = node.items
+    if not items or not isinstance(items[0], Token):
+        return None
+    net = items[0].text
+    for sub in items[1:]:
+        if isinstance(sub, SExp):
+            shape = read_shape(sub)
+            if shape is not None:
+                return Plane(net=net, shape=shape)
+    return None