fix: prevent self-collision on rapid multi-key input using pending direction buffer

Snake.Core/Snake.cs

@@ -3,6 +3,7 @@ namespace Snake.Core;
 public sealed class Snake
 {
     private readonly LinkedList<Position> _segments = new();
+    private Direction _pendingDirection;
 
     public Snake(Position start, int length, Direction direction)
     {
@@ -10,6 +11,7 @@ public sealed class Snake
             throw new ArgumentOutOfRangeException(nameof(length), "Length must be at least 1.");
 
         Direction = direction;
+        _pendingDirection = direction;
 
         var offset = DirectionToOffset(direction);
         for (var i = 0; i < length; i++)
@@ -28,23 +30,27 @@ public sealed class Snake
 
     public void SetDirection(Direction direction)
     {
-        if (direction == Direction)
+        if (direction == _pendingDirection)
             return;
 
         if (AreOpposite(Direction, direction))
             return;
 
-        Direction = direction;
+        if (_pendingDirection != Direction && AreOpposite(_pendingDirection, direction))
+            return;
+
+        _pendingDirection = direction;
     }
 
     public Position PeekNextHead()
     {
-        var offset = DirectionToOffset(Direction);
+        var offset = DirectionToOffset(_pendingDirection);
         return new Position(Head.X + offset.X, Head.Y + offset.Y);
     }
 
     public Position Move(bool grow = false)
     {
+        Direction = _pendingDirection;
         var newHead = PeekNextHead();
 
         _segments.AddFirst(newHead);