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/model.py

@@ -170,6 +170,14 @@ class NetClass:
     via_rule: str | None = None
 
 
+@dataclass
+class Plane:
+    """A ``(plane <net> <shape>)``: a copper zone of that net (KiCad's zones)."""
+
+    net: str
+    shape: Shape
+
+
 @dataclass
 class Keepout:
     """A structure-level keepout area with its kind."""
@@ -187,6 +195,8 @@ class DsnBoard:
     resolution: Resolution = field(default_factory=Resolution)
     unit: str = "mil"
     layers: list[Layer] = field(default_factory=list)
+    #: copper zones, as planes of a net
+    planes: list[Plane] = field(default_factory=list)
     #: bounding-box boundary shape (layer ``pcb``), if present
     boundary: Shape | None = None
     #: outline shapes (layer ``signal``)