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/search_tree.py

@@ -47,6 +47,9 @@ class TreeShape:
     routed: bool = False
     exact: Simplex | None = None
     seg: tuple | None = None  # (a: IntPoint, b: IntPoint, half_width: int)
+    #: which copper a keepout tile stops: "all", "wires" (a wire_keepout,
+    #: vias allowed) or "vias" (a via_keepout)
+    blocks: str = "all"
 
     def on_layer(self, layer: int) -> bool:
         return layer in self.layers
@@ -118,7 +121,9 @@ class ShapeSearchTree:
                     result.append(shape)
         return result
 
-    def clearance_conflict(self, box: IntBox, net_no: int, layer: int, clearance: int) -> bool:
+    def clearance_conflict(
+        self, box: IntBox, net_no: int, layer: int, clearance: int, kind: str = "wire"
+    ) -> bool:
         """True if placing ``box`` (of ``net_no`` on ``layer``) would come *closer
         than* ``clearance`` to a different net's tile on that layer (exact).
 
@@ -129,6 +134,8 @@ class ShapeSearchTree:
         for shape in self.overlapping(expanded):
             if shape.net_no == net_no or not shape.on_layer(layer):
                 continue
+            if shape.blocks != "all" and shape.blocks != kind + "s":
+                continue  # a wire-only keepout does not stop a via, and so on
             if shape.exact is not None:
                 if overlaps_2d(shape.exact, expanded):
                     return True