LONDON — HSBC's London-listed shares tumbled over 6% when the markets opened on Thursday after a privatization bid for its Hong Kong–based subsidiary Hang Seng Bank.The pan-European Stoxx edged lower after the opening bell, down 0.15% as of 8:30 a.m. (3:30 a.m. ET).Europe's largest lender, HSBC, put forward a privatization proposal to shareholders for Hang Seng on Thursday. It is a controlled shareholder of the bank with a 63% stake."If approved, Hang Seng will become a wholly owned subsidiary of HSBC Asia ...