By Katanga Johnson and John McCrank WASHINGTON/NEW YORK (Reuters) -The top U.S. securities regulator on Wednesday unveiled potential rule changes to transform how Wall Street handles retail stock trades after the meme stock mania last year raised questions about whether mom-and-pop investors were getting the best price. The rules, spelled out by U.S. Securities and Exchange Commission chair Gary Gensler, would require trading firms to directly compete to execute trades from retail investors to boost competition. The Wall Street watchdog plans to scrutinize the controversial payment for order f…