Player/Player.Undo.cs

Partial Player class adding an Undo field that references the per-player undo stack from UndoSystem using the network owner's SteamId.

Networking
public sealed partial class Player
{
	/// <summary>
	/// Access the undo system for this player
	/// </summary>
	public UndoSystem.PlayerStack Undo => UndoSystem.Current.For( Network.Owner.SteamId );
}