fix: retain notifications while the scene background is absent

Buffer reward messages until the shell returns without allocating detached UI nodes. Register mod tests that verify notification ordering and prevent duplicates.

tests/MessagesHelperTests.cs

@@ -0,0 +1,51 @@
+namespace Cthangover.Mods.Interface.Messages.Tests
+{
+    using System.Collections.Generic;
+    using Cthangover.Core.Mods.Testing;
+    using Cthangover.Core.Scenes;
+    using Cthangover.Core.UI.Messages;
+    using Godot;
+
+    /// <summary>Battle/menu notifications must survive a missing shell without allocating orphan UI trees.</summary>
+    public class MessagesHelperTests
+    {
+        [ModTest("messages/without_background_preserves_each_notification")]
+        static void WithoutBackground(ModTestContext ctx)
+        {
+            var root = SceneContextNode.Instance.GetTree().Root;
+            var background = root.FindChild("Background", true, false) as Control;
+            var originalName = background?.Name ?? default;
+            var grouped = background?.IsInGroup("Background") == true;
+            var previous = new List<string>();
+            while (NotificationBuffer.TryDequeue(out var pendingText)) previous.Add(pendingText);
+            try
+            {
+                // The shared test scene has a background; model a battle without detaching or freeing borrowed UI.
+                if (background != null)
+                {
+                    background.Name = "MessagesContractBackground";
+                    if (grouped) background.RemoveFromGroup("Background");
+                }
+                ModAssert.True(root.FindChild("Background", true, false) == null, "fixture has no shell background");
+                for (var i = 0; i < 40; i++) MessagesHelper.AddMessage("reward/" + i);
+                ModAssert.Equal(40, NotificationBuffer.PendingCount, "every late reward is buffered exactly once");
+                for (var i = 0; i < 40; i++)
+                {
+                    ModAssert.True(NotificationBuffer.TryDequeue(out var actual), "notification is retained");
+                    ModAssert.Equal("reward/" + i, actual, "text and order survive until the shell returns");
+                }
+                ModAssert.Equal(0, NotificationBuffer.PendingCount, "no duplicate banners remain");
+            }
+            finally
+            {
+                if (background != null)
+                {
+                    background.Name = originalName;
+                    if (grouped) background.AddToGroup("Background");
+                }
+                NotificationBuffer.Clear();
+                foreach (var previousText in previous) NotificationBuffer.Add(previousText);
+            }
+        }
+    }
+}