public class AuditableBuilder : IOrderBuilder
{
    private readonly IOrderBuilder _inner;
    public AuditableBuilder(IOrderBuilder inner) => _inner = inner;
    public IOrderBuilder AddItem(string sku)
    {
        Log($"Add {sku}");
        return _inner.AddItem(sku);
    }
    public Order Build() => _inner.Build();
}