port: P3-c PythonExchange + exchange module (1:1 40250)

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>

extension/src/platform/ScriptLib/PythonBoot.cpp

@@ -349,6 +349,7 @@ bool run_main_script_body(const char* lpCmdLine, std::string* error)
 	initPlayer();
 	initItem();
 	initNonPlayer();
+	initTrade();
 	initChat();
 	initnet();
 	initeffect();