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/items.py

@@ -72,6 +72,10 @@ class ObstacleArea(Item):
     name: str | None = None
     layer: int = 0
     tiles: list = field(default_factory=list)
+    #: a DSN keepout blocks both; wire_keepout only traces (KiCad's "no
+    #: tracks" rule area, where vias are allowed), via_keepout only vias
+    blocks_wires: bool = True
+    blocks_vias: bool = True
 
     def is_obstacle(self) -> bool:
         return True
@@ -87,7 +91,16 @@ class ObstacleArea(Item):
 
 @dataclass
 class ConductionArea(ObstacleArea):
-    """A power-plane conduction area (an obstacle carrying a net)."""
+    """A copper zone of one net (a DSN plane): not an obstacle to routing,
+    since KiCad refills it around other nets' copper, but copper of its net:
+    pads it touches are connected through it, and a route of its net ends on
+    it by placing a via inside it."""
+
+    #: the zone outline, for point containment
+    polygon: object = None
+
+    def is_obstacle(self) -> bool:
+        return False
 
 
 @dataclass