Copy PythonExchange.cpp/PythonExchangeModule.cpp verbatim, drop the pending
stand-in and the exchange module stub, register initTrade() after
initNonPlayer(). The fake server answers CG_EXCHANGE START with the partner's
item + gold and CANCEL with END; port.login_flow drives the exchange round-trip
through the Python exchange/net modules and uiexchange.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
