Add scene transitions (fade, wipe) to SceneManager

Switch(scene, transition) covers the old scene, swaps at full coverage
(hiding slow OnLoad), then reveals the new one. Built-in Fade and Wipe
transitions draw through TransitionRenderer (BasicEffect quad, no
SpriteBatch); custom transitions subclass Transition. Runs on unscaled
time so it works while gameplay is paused; Switch during a transition
replaces the pending target.

Sample: Tab now fades into the stress scene and wipes back.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

samples/MrGameEng.Sample/SampleSystems.cs

@@ -82,9 +82,9 @@ public sealed class BounceSystem(RectF bounds) : QuerySystem<Transform2D, Veloci
     }
 }
 
-/// <summary>Пауза (P), музыка (M), переключение сцены (Tab).</summary>
+/// <summary>Пауза (P), музыка (M), переключение сцены с переходом (Tab).</summary>
 public sealed class SceneHotkeysSystem(
-    EngineContext context, ActionMap<SampleAction> actions, Func<Scene> nextScene) : BaseSystem
+    EngineContext context, ActionMap<SampleAction> actions, Func<Scene> nextScene, Transition transition) : BaseSystem
 {
     protected override void OnUpdateGroup()
     {
@@ -106,9 +106,9 @@ public sealed class SceneHotkeysSystem(
             }
         }
 
-        if (actions.IsPressed(SampleAction.SwitchScene))
+        if (actions.IsPressed(SampleAction.SwitchScene) && !context.Scenes.IsTransitioning)
         {
-            context.Scenes.Switch(nextScene());
+            context.Scenes.Switch(nextScene(), transition);
         }
     }
 }