test(capture): fixtures check client game messages, with a synthetic fixture

The fixture check now also reads every recorded client game message with the
struct the server reads it with (GameMessageHandler::CreateReceived) and writes
it again: it must read the whole message and give back the same bits. Game
message fields are decoded with the server's structs in the fixture tests.

A synthetic fixture, built in the test from the server's own structs (login,
position update, a client game message), goes through the export steps
(portable, anonymised, saved, read again) and passes the same checks; recorded
fixtures stay in tests/fixtures-local and are never committed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

dGame/dGameMessages/GameMessageHandler.h

@@ -23,6 +23,10 @@
 
 namespace GameMessageHandler {
 	void HandleMessage(RakNet::BitStream& inStream, const SystemAddress& sysAddr, LWOOBJID objectID, MessageType::Game messageID);
+
+	// A new, empty typed struct for a message a client sends, or nullptr when the server reads it inline (capture
+	// fixtures read recorded messages with it)
+	std::unique_ptr<GameMessages::NetGameMsg> CreateReceived(MessageType::Game messageID);
 };
 
 #endif // GAMEMESSAGEHANDLER_H