Event-driven dialog queue, story flag command, mod unit tests

Push-based event chains, runtime fixes (single end, goto lifecycle, if,
busy box, held delay, empty options, timed/animated/effect actions), the
'flag name value' DSL command, autoplay --dialogs=N, 19 mod unit tests.

tests/ActionEffectTests.cs

@@ -0,0 +1,44 @@
+namespace Cthangover.Mods.Dialog.Tests
+{
+    using System.Threading.Tasks;
+    using Cthangover.Core.Mods.Testing;
+    using Cthangover.Mods.Dialog.Action.Impls;
+    using Godot;
+
+    /// <summary>
+    /// <see cref="ActionEffect"/> shows its effect: the instance lands in the ViewBox UI
+    /// layer when the action runs and is freed when the dialog ends.
+    /// </summary>
+    public class ActionEffectTests
+    {
+        [ModTest("effect/spawned_into_view_until_dialog_end", TimeoutSeconds = 10)]
+        static async Task SpawnedIntoViewUntilDialogEnd(ModTestContext ctx)
+        {
+            var stage = await DialogStage.Create();
+            var view = stage.AddViewBox();
+            var template = new Node2D { Name = "TestEffect" };
+            var prefab = new PackedScene();
+            prefab.Pack(template);
+            template.Free();
+            try
+            {
+                var dlg = new DialogQueue();
+                dlg.Queue.Add(new ActionEffect { Prefab = prefab });
+                dlg.Queue.Add(new ProbeAction());
+
+                stage.Box.SetDialogQueueAndRun(dlg, null, 0, null);
+                var layer = view.GetInteractiveLayer("ui");
+                ModAssert.Equal(1, layer.GetChildCount(), "effect is shown in the UI layer");
+                ModAssert.Equal("TestEffect", layer.GetChild(0).Name.ToString(), "the shown node is the effect");
+
+                stage.Box.NextAction();
+                await DialogStage.Frames(2);
+                ModAssert.Equal(0, layer.GetChildCount(), "effect is freed when the dialog ends");
+            }
+            finally
+            {
+                stage.Free();
+            }
+        }
+    }
+}