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/board/board.py

@@ -12,7 +12,7 @@ from __future__ import annotations
 from freeroute.geometry import IntBox
 
 from .clearance import ClearanceMatrix
-from .items import Item, ObstacleArea, Pin, Trace, Via
+from .items import ConductionArea, Item, ObstacleArea, Pin, Trace, Via
 from .layer import LayerStructure
 from .net import Nets
 
@@ -57,7 +57,15 @@ class BasicBoard:
         return [it for it in self._items if isinstance(it, Pin)]
 
     def get_obstacle_areas(self) -> list[ObstacleArea]:
-        return [it for it in self._items if isinstance(it, ObstacleArea)]
+        """Keepouts. Copper zones are not obstacles; see get_conduction_areas."""
+        return [
+            it
+            for it in self._items
+            if isinstance(it, ObstacleArea) and not isinstance(it, ConductionArea)
+        ]
+
+    def get_conduction_areas(self) -> list[ConductionArea]:
+        return [it for it in self._items if isinstance(it, ConductionArea)]
 
     def get_traces(self) -> list[Trace]:
         return [it for it in self._items if isinstance(it, Trace)]