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/SceneEventControllerTests.cs

@@ -0,0 +1,68 @@
+namespace Cthangover.Mods.Dialog.Tests
+{
+    using System.Collections.Generic;
+    using Cthangover.Core.Mods.Testing;
+    using Cthangover.Core.UI.Event;
+
+    /// <summary>
+    /// Dispatch contract of the core <see cref="SceneEventController"/> (tested here:
+    /// it is a Godot node, xUnit cannot create it; the dialog actions are its main
+    /// listeners): lower priority first, registration order within a priority, and a
+    /// listener list changed during a dispatch neither skips nor double-calls anyone.
+    /// </summary>
+    public class SceneEventControllerTests
+    {
+        [ModTest("event_controller/lower_priority_first_then_registration_order")]
+        static void LowerPriorityFirst(ModTestContext ctx)
+        {
+            var log = new List<string>();
+            var controller = new SceneEventController();
+            try
+            {
+                controller.AddUpdateEventListener(new OrderListener { Name = "b", Priority = 1, Log = log });
+                controller.AddUpdateEventListener(new OrderListener { Name = "a", Priority = 0, Log = log });
+                controller.AddUpdateEventListener(new OrderListener { Name = "c", Priority = 1, Log = log });
+
+                controller._Process(0.0);
+
+                ModAssert.Equal("a,b,c", string.Join(",", log), "dispatch order");
+            }
+            finally
+            {
+                controller.Free();
+            }
+        }
+
+        [ModTest("event_controller/list_changed_during_dispatch")]
+        static void ListChangedDuringDispatch(ModTestContext ctx)
+        {
+            var log = new List<string>();
+            var controller = new SceneEventController();
+            try
+            {
+                var removed = new OrderListener { Name = "removed", Priority = 1, Log = log };
+                var added = new OrderListener { Name = "added", Priority = 0, Log = log };
+                var changer = new OrderListener
+                {
+                    Name = "changer", Priority = 0, Log = log, Controller = controller,
+                    RemoveOnUpdate = removed, AddOnUpdate = added,
+                };
+                var last = new OrderListener { Name = "last", Priority = 2, Log = log };
+                controller.AddUpdateEventListener(changer);
+                controller.AddUpdateEventListener(removed);
+                controller.AddUpdateEventListener(last);
+
+                controller._Process(0.0);
+                ModAssert.Equal("changer,last", string.Join(",", log), "first dispatch: removed one skipped, added one waits");
+
+                log.Clear();
+                controller._Process(0.0);
+                ModAssert.Equal("changer,added,last", string.Join(",", log), "second dispatch includes the added listener");
+            }
+            finally
+            {
+                controller.Free();
+            }
+        }
+    }
+}