Explain the greyed-out ready box and count cars in the depot

A disabled checkbox shows no tooltip, so the settings override that greys
out "ready to go" is now said under it. The consist vehicle count counted
units, so a multi-unit was one vehicle; it now counts cars like the stats bar.

StarterNG.Tests/ConsistTailTests.cs

@@ -21,6 +21,17 @@ public class ConsistTailTests
         _consist = new Consist(_catalog, new VehicleInfo(_catalog, installation.Physics));
     }
 
+    [Fact]
+    public void The_vehicle_count_counts_cars_not_units()
+    {
+        Append("b111-01");
+        var pair = Append("b111-02");
+        pair.Cars.Add(pair.Cars[0].Clone());
+
+        Assert.Equal(2, _consist.Count);
+        Assert.Equal(3, _consist.VehicleCount);
+    }
+
     [Fact]
     public void A_new_last_car_is_left_uncoupled_at_the_back()
     {