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>

tests/MrGameEng.Core.Tests/SceneTransitionTests.cs

@@ -0,0 +1,108 @@
+using MrGameEng.Core;
+using Xunit;
+
+namespace MrGameEng.Core.Tests;
+
+public class SceneTransitionTests
+{
+    private sealed class TrackingScene : Scene
+    {
+        public int LoadCount;
+        public int UnloadCount;
+
+        protected override void OnLoad() => LoadCount++;
+
+        protected override void OnUnload() => UnloadCount++;
+    }
+
+    private static void Tick(EngineContext context, float seconds)
+    {
+        context.Clock.Advance(seconds);
+        context.Scenes.Update(context.Clock);
+    }
+
+    [Fact]
+    public void TransitionSwitch_KeepsOldScene_UntilFullyCovered()
+    {
+        var context = new EngineContext();
+        var first = new TrackingScene();
+        var second = new TrackingScene();
+        context.Scenes.Switch(first);
+        Tick(context, 0.016f);
+
+        // Fade(1.0) → фаза закрытия 0.5 c, фаза открытия 0.5 c.
+        context.Scenes.Switch(second, Transition.Fade(1f));
+        Tick(context, 0.2f);
+
+        Assert.True(context.Scenes.IsTransitioning);
+        Assert.Same(first, context.Scenes.Current);
+        Assert.Equal(0, second.LoadCount);
+
+        Tick(context, 0.4f); // суммарно 0.6 > 0.5 — экран закрыт, своп произошёл
+
+        Assert.Same(second, context.Scenes.Current);
+        Assert.Equal(1, first.UnloadCount);
+        Assert.Equal(1, second.LoadCount);
+        Assert.True(context.Scenes.IsTransitioning); // идёт фаза открытия
+
+        Tick(context, 0.6f); // открытие завершено
+
+        Assert.False(context.Scenes.IsTransitioning);
+        Assert.Same(second, context.Scenes.Current);
+    }
+
+    [Fact]
+    public void Transition_UsesUnscaledTime_WorksWhilePaused()
+    {
+        var context = new EngineContext();
+        var first = new TrackingScene();
+        var second = new TrackingScene();
+        context.Scenes.Switch(first);
+        Tick(context, 0.016f);
+        context.Clock.TimeScale = 0f; // игра на паузе
+
+        context.Scenes.Switch(second, Transition.Fade(0.2f));
+        Tick(context, 0.15f);
+        Tick(context, 0.15f);
+
+        Assert.Same(second, context.Scenes.Current);
+        Assert.False(context.Scenes.IsTransitioning);
+    }
+
+    [Fact]
+    public void SwitchDuringTransition_ReplacesPendingTarget()
+    {
+        var context = new EngineContext();
+        var first = new TrackingScene();
+        var second = new TrackingScene();
+        var third = new TrackingScene();
+        context.Scenes.Switch(first);
+        Tick(context, 0.016f);
+
+        context.Scenes.Switch(second, Transition.Fade(1f));
+        Tick(context, 0.1f);
+        context.Scenes.Switch(third); // передумали, пока экран закрывается
+
+        Tick(context, 0.5f);
+
+        Assert.Same(third, context.Scenes.Current);
+        Assert.Equal(0, second.LoadCount);
+    }
+
+    [Fact]
+    public void ZeroDurationTransition_SwapsOnNextUpdates()
+    {
+        var context = new EngineContext();
+        var first = new TrackingScene();
+        var second = new TrackingScene();
+        context.Scenes.Switch(first);
+        Tick(context, 0.016f);
+
+        context.Scenes.Switch(second, Transition.Fade(0f));
+        Tick(context, 0.016f);
+        Tick(context, 0.016f);
+
+        Assert.Same(second, context.Scenes.Current);
+        Assert.False(context.Scenes.IsTransitioning);
+    }
+}