fix: smooth tail interpolation

When snake doesn't grow, the last segment (new tail) should interpolate
from the old tail's position (previous[segmentIndex]), not from the
second-to-last segment (previous[segmentIndex-1]) which didn't move.

Snake.Avalonia/Views/AnimationHelper.cs

@@ -75,9 +75,11 @@ public static class AnimationHelper
         else
         {
             // When not growing: tail removed, new head added.
-            // current[0] = new head, current[1] = old head, etc.
+            // current[0] = new head, current[1] = old head, … current[N-2] = old[N-3]
-            // current[i] (i>=1) was at old[i-1]
+            // BUT the last segment (tail): it was old[N-1] (the removed tail's position)
-            var prevIdx = segmentIndex - 1;
+            // because visually it slides from where the old tail was.
+            var isLast = segmentIndex == currentSegments.Count - 1;
+            var prevIdx = isLast ? segmentIndex : segmentIndex - 1;
             if (prevIdx < previousSegments.Count)
                 return previousSegments[prevIdx];
         }