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>

src/MrGameEng.Core/Transition.cs

@@ -0,0 +1,73 @@
+using Microsoft.Xna.Framework;
+
+namespace MrGameEng.Core;
+
+/// <summary>Phase of a scene transition.</summary>
+public enum TransitionPhase
+{
+    /// <summary>The old scene is being covered (coverage grows 0 → 1).</summary>
+    Out,
+
+    /// <summary>The new scene is being revealed (coverage shrinks 1 → 0).</summary>
+    In,
+}
+
+/// <summary>
+/// Visual transition between scenes. The scene switch itself happens at full coverage,
+/// so a slow <c>OnLoad</c> of the next scene is hidden behind the overlay.
+/// Transitions are stateless and reusable; progress is tracked by <see cref="SceneManager"/>.
+/// Runs on unscaled time, so it works while gameplay is paused.
+/// </summary>
+public abstract class Transition
+{
+    /// <summary>Seconds the covering phase takes.</summary>
+    public float OutDuration { get; }
+
+    /// <summary>Seconds the revealing phase takes.</summary>
+    public float InDuration { get; }
+
+    /// <summary>Creates a transition with explicit phase durations.</summary>
+    protected Transition(float outDuration, float inDuration)
+    {
+        OutDuration = Math.Max(0f, outDuration);
+        InDuration = Math.Max(0f, inDuration);
+    }
+
+    /// <summary>
+    /// Draws the overlay. <paramref name="coverage"/> is 0 (scene fully visible) to
+    /// 1 (scene fully covered); <paramref name="phase"/> tells which side of the switch this is.
+    /// </summary>
+    public abstract void Draw(TransitionRenderer renderer, float coverage, TransitionPhase phase);
+
+    /// <summary>Fade through a solid color (black by default). Total duration is split between out and in.</summary>
+    public static Transition Fade(float duration = 0.6f, Color? color = null) =>
+        new FadeTransition(duration / 2f, duration / 2f, color ?? Color.Black);
+
+    /// <summary>A curtain wiping across the screen (black by default). Total duration is split between out and in.</summary>
+    public static Transition Wipe(float duration = 0.6f, Color? color = null) =>
+        new WipeTransition(duration / 2f, duration / 2f, color ?? Color.Black);
+
+    private sealed class FadeTransition(float outDuration, float inDuration, Color color)
+        : Transition(outDuration, inDuration)
+    {
+        public override void Draw(TransitionRenderer renderer, float coverage, TransitionPhase phase) =>
+            renderer.Fill(0f, 0f, 1f, 1f, color, coverage);
+    }
+
+    private sealed class WipeTransition(float outDuration, float inDuration, Color color)
+        : Transition(outDuration, inDuration)
+    {
+        public override void Draw(TransitionRenderer renderer, float coverage, TransitionPhase phase)
+        {
+            // Out: шторка растёт слева направо; In: уезжает дальше вправо.
+            if (phase == TransitionPhase.Out)
+            {
+                renderer.Fill(0f, 0f, coverage, 1f, color);
+            }
+            else
+            {
+                renderer.Fill(1f - coverage, 0f, coverage, 1f, color);
+            }
+        }
+    }
+}