Count on a zone's connectivity only when no track can cross it

A zone outline is where copper may go, not where it ends up: on the rc4
sense board the board-wide GND pours break into islands around other
nets' tracks, and treating every pad inside them as connected left 28
GND islands unjoined (63 unconnected against 42 without zones). Trust a
zone only when a no-tracks rule area covers it on its layer, as the
power stage's pours are covered; route the rest with tracks as before.

rc4, exact engine, resolution 2: power stage 5 unconnected (JAR 12),
sense board 41 (JAR 32), no DRC errors on either.

src/freeroute/route/grid_router.py

@@ -506,6 +506,23 @@ class GridRouter:
             self._zone_cells[key] = sorted(set(nodes))
         return self._zone_cells[key]
 
+    def _zone_is_solid(self, zone, coverage: float = 0.95) -> bool:
+        """Whether a zone's fill can be trusted to be one piece of copper.
+
+        A zone outline is where copper may go, not where it will be: other
+        nets' tracks through a board-wide pour break its fill into islands,
+        and a pad inside the outline need not reach the rest of the net. It
+        is safe to count on only when no track can cross it, which is what a
+        no-tracks rule area over the zone guarantees (the rc4 power pours).
+        Otherwise the zone is left out and its pads are routed with tracks.
+        """
+        cells = [cell for tile in zone.tiles for cell in self._cells_in_box(tile.bounding_box(), 0)
+                 if zone.polygon.contains(self._cell_center(cell))]
+        if not cells:
+            return False
+        covered = sum((*cell, zone.layer) in self._keepout for cell in cells)
+        return covered >= coverage * len(cells)
+
     def _group_ends(self, group, net_no: int, conns) -> dict[Node, Pin | None]:
         """End nodes of a group, each mapped to its pin (None for a zone cell)."""
         ends: dict[Node, Pin | None] = {}
@@ -693,7 +710,7 @@ class GridRouter:
         pins_by_net = self._pins_by_net()
         zones_by_net: dict[int, list] = {}
         for zone in self.board.get_conduction_areas():
-            if zone.layer in self.layers and zone.net_nos:
+            if zone.layer in self.layers and zone.net_nos and self._zone_is_solid(zone):
                 zones_by_net.setdefault(zone.net_nos[0], []).append(zone)
         conns: list[_Connection] = []
         nets = set(pins_by_net) | set(zones_by_net)