Rich in works by Old Masters and American impressionists, the palazzo-like Isabella Stewart Gardner Museum says its investment portfolio closed out the year valued at $161.6 million — some 53 percent higher than at the end of June, the close of its last fiscal year. A rise in the stock market and some $25 million in grants and gifts gave the balance sheet a boost in time for building a planned $118 million expansion designed by Renzo Piano Building Workshop.