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.

tests/route/test_real_board_geometry.py

@@ -138,3 +138,13 @@ def test_a_pad_reaches_a_zone_under_a_no_tracks_area_by_a_via():
         if layer == f_cu:  # no track runs inside the no-tracks area
             assert not any(t.contains(p) and not t.bounding_box().is_empty()
                            for t in pour.tiles for p in pts[1:-1])
+
+
+def test_a_pour_other_nets_can_cross_is_not_trusted_to_connect():
+    # the same zone with no no-tracks area over it: its fill could be split by
+    # other nets' tracks, so its pads are joined by routing, not assumed
+    places = "(place P1 5500 -7000 front 0) (place P2 8500 -7000 front 0)"
+    dsn = parse_dsn(_dsn(f"(component P {places})", "(net N (pins P1-1 P2-1))", _ZONE))
+    result, _, _ = route_dsn_board_exact(dsn)
+    net = next(iter(result.routed_net_numbers))
+    assert result.result.wires.get(net)  # P1 and P2 got a track between them