feat: smooth snake animation + beautiful Avalonia UI overhaul

- Two-timer architecture: game timer + 60fps render timer for smooth interpolation
- Snake body: StreamGeometry path with teal gradient, rounded joins
- Directional head with white eyes and dark pupils
- Food: pulsating glow, highlight, green leaf animation
- Modern dark theme (#0D1117), glassmorphism HUD
- Speed indicator bar, score +N popup
- High score persistence to JSON
- All keyboard shortcuts: Arrows, WASD, Space/P pause, Enter start, R restart, Esc quit
- Window resizable, 640x540 default

New files: AnimationHelper.cs, HighScoreManager.cs, SnakeRenderer.cs

Snake.Core/Snake.cs

@@ -15,18 +15,28 @@ public sealed class Snake
         var offset = DirectionToOffset(direction);
         for (var i = 0; i < length; i++)
         {
-            _segments.AddLast(new Position(
+            var pos = new Position(
                 start.X - offset.X * i,
-                start.Y - offset.Y * i));
+                start.Y - offset.Y * i);
+            _segments.AddLast(pos);
+            _previousSegments.Add(pos);
         }
     }
 
+    private readonly List<Position> _previousSegments = new();
+
     public Direction Direction { get; private set; }
 
     public Position Head => _segments.First!.Value;
 
     public IReadOnlyCollection<Position> Segments => _segments;
 
+    /// <summary>
+    /// Segment positions before the last Move() call, used for interpolation.
+    /// Empty (Count == 0) before first move.
+    /// </summary>
+    public IReadOnlyList<Position> PreviousSegments => _previousSegments;
+
     /// <summary>
     /// Enqueues a direction change to be applied on a future tick.
     /// </summary>
@@ -56,6 +66,11 @@ public sealed class Snake
         if (_pendingDirections.Count > 0)
             Direction = _pendingDirections.Dequeue();
 
+        // Capture current segments as "previous" for interpolation
+        _previousSegments.Clear();
+        foreach (var seg in _segments)
+            _previousSegments.Add(seg);
+
         var offset = DirectionToOffset(Direction);
         var newHead = new Position(Head.X + offset.X, Head.Y + offset.Y);