Move the battery option to the depot as a consist property

It was a global preference that rewrote the trainset speed at launch, while what
it really sets is whether this consist starts prepared or cold. Now a checkbox in
the consist-scope section, acting on the consist being edited.

StarterNG.Tests/TrainsetReadyTests.cs

@@ -0,0 +1,56 @@
+using StarterNG.Classes;
+
+namespace StarterNG.Tests;
+
+public class TrainsetReadyTests
+{
+    private static Trainset Parse(string velocity) =>
+        new($"trainset pociag tor_1 10 {velocity}\n" +
+            "node -1 -1 ep07-001 dynamic PKP/EP07 ep07 ep07.mmd 0 headdriver 0\n" +
+            "endtrainset");
+
+    [Fact]
+    public void A_consist_authored_standing_still_is_not_ready()
+    {
+        Assert.False(Parse("0").ReadyToGo);
+    }
+
+    [Fact]
+    public void Marking_a_standing_consist_ready_gives_it_the_prepared_creep()
+    {
+        var trainset = Parse("0");
+
+        trainset.ReadyToGo = true;
+
+        Assert.True(trainset.ReadyToGo);
+        Assert.Equal(0.1f, trainset.Velocity, 3);
+    }
+
+    [Fact]
+    public void A_consist_authored_on_the_move_is_ready_and_keeps_its_speed()
+    {
+        var trainset = Parse("40");
+
+        Assert.True(trainset.ReadyToGo);
+
+        trainset.ReadyToGo = false;
+        Assert.Equal(0f, trainset.Velocity);
+
+        trainset.ReadyToGo = true;
+        Assert.Equal(40f, trainset.Velocity);
+    }
+
+    [Fact]
+    public void A_consist_running_the_other_way_keeps_its_direction()
+    {
+        // The sign of the speed is what DynObj.cpp turns into the running
+        // direction, so restoring readiness must not flip it.
+        var trainset = Parse("-40");
+
+        trainset.ReadyToGo = false;
+        trainset.ReadyToGo = true;
+
+        Assert.Equal(-40f, trainset.Velocity);
+        Assert.True(trainset.ReadyToGo);
+    }
+}