Add DelayMs to ElementTransition for per-element stagger

Add optional DelayMs parameter to the ElementTransition record.
When set, the CCV applies DelayTime/SetInitialValueBeforeDelay to
the per-element progress KeyFrameAnimation so callers can stagger
layout transitions by element index.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: arcadiogarcia

src/CompositionCollectionView/CompositionCollectionLayout.Transition.cs

@@ -162,6 +162,11 @@ void TransferElements()
                     var progressAnimation = Compositor.CreateScalarKeyFrameAnimation();
                     progressAnimation.Duration = TimeSpan.FromMilliseconds(transition.Length);
                     progressAnimation.StopBehavior = AnimationStopBehavior.SetToFinalValue;
+                    if (transition.DelayMs > 0)
+                    {
+                        progressAnimation.DelayTime = TimeSpan.FromMilliseconds(transition.DelayMs);
+                        progressAnimation.DelayBehavior = AnimationDelayBehavior.SetInitialValueBeforeDelay;
+                    }
                     progressAnimation.InsertKeyFrame(0, 0f, transition.EasingFunction);
                     progressAnimation.InsertKeyFrame(1, 1f, transition.EasingFunction);
 

src/CompositionCollectionView/CompositionCollectionLayout.Update.cs

@@ -239,6 +239,11 @@ void UpdateAndTransitionElement(ElementReference<TId, TItem> element, TItem newD
                 var progressAnimation = Compositor.CreateScalarKeyFrameAnimation();
                 progressAnimation.Duration = TimeSpan.FromMilliseconds(transition.Length);
                 progressAnimation.StopBehavior = AnimationStopBehavior.SetToFinalValue;
+                if (transition.DelayMs > 0)
+                {
+                    progressAnimation.DelayTime = TimeSpan.FromMilliseconds(transition.DelayMs);
+                    progressAnimation.DelayBehavior = AnimationDelayBehavior.SetInitialValueBeforeDelay;
+                }
                 progressAnimation.InsertKeyFrame(0, 0f, transition.EasingFunction);
                 progressAnimation.InsertKeyFrame(1, 1f, transition.EasingFunction);
 

src/CompositionCollectionView/CompositionCollectionLayout.cs

@@ -133,7 +133,7 @@ public void StopElementAnimation(ElementReference<TId, TItem> element)
         element.Visual.StopAnimation(AnimationConstants.Orientation);
     }
 
-    public record ElementTransition(uint Length, CompositionEasingFunction EasingFunction);
+    public record ElementTransition(uint Length, CompositionEasingFunction EasingFunction, uint DelayMs = 0);
 
     protected virtual void Dispose(bool disposing)
     {