Minor optimizations

Solver/Crafty/Arena.cs

@@ -6,24 +6,23 @@ public class Arena<T> where T : struct
 {
     public readonly record struct Node
     {
-        public int? Parent { get; init; }
-        public int Index { get; init; }
-        public List<int> Children { get; init; }
         public T State { get; init; }
+        public List<int> Children { get; init; }
+        public int Parent { get; init; }
     }
 
     private readonly List<Node> nodes = new();
 
     public Arena(T initialState = default)
     {
-        nodes.Add(new() { Parent = null, Index = 0, Children = new(), State = initialState });
+        nodes.Add(new() { Parent = -1, Children = new(), State = initialState });
     }
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public int Insert(int parentIndex, T state)
     {
         var index = nodes.Count;
-        nodes.Add(new() { Parent = parentIndex, Index = index, Children = new(), State = state });
+        nodes.Add(new() { Parent = parentIndex, Children = new(), State = state });
         nodes[parentIndex].Children.Add(index);
         return index;
     }