The state transportation department on Thursday will hold a public hearing on a design proposal to replace the historic Cradock Bridge in Medford Square. The stone-arch bridge, dating to 1637, is a key entry point to downtown. The bridge, which carries Main Street over the Mystic River, will be reconstructed in phases, likely starting in spring 2014. The project, estimated to cost $10 million to $12.7 million, will be paid for with state funding. The hearing will be held at 7 p.m. in the council chamber at City Hall.