fix: fix multiplayer player data mix between different players bug

Fixes a Win64 multiplayer issue where player data (`players/*.dat`) could be mismatched because identity was effectively tied to connection-order `smallId` XUIDs.
Introduces a deterministic username-derived persistent XUID and integrates it into the existing XUID-based save pipeline.

- Added `Windows64_NameXuid` for deterministic `name -> persistent xuid` resolution
- On Win64 login (`PlayerList`), set `ServerPlayer::xuid` from username-based resolver
- Aligned local player `xuid` assignment (`Minecraft`) for create/init/respawn paths to use the same resolver
- Added Win64 local-self guard in `ClientConnection::handleAddPlayer` using name match to avoid duplicate local remote-player creation
- Kept `IQNet::GetPlayerByXuid` compatibility fallback behavior, while extending lookup to also resolve username-based XUIDs
- Moved implementation to `Minecraft.Client/Windows64/Windows64_NameXuid.h`; kept legacy `Win64NameXuid.h` as compatibility include

Rename migration is intentionally out of scope (same-name identity only).

Minecraft.Client/PlayerList.cpp

@@ -16,6 +16,7 @@
 #include "..\Minecraft.World\ArrayWithLength.h"
 #include "..\Minecraft.World\net.minecraft.network.packet.h"
 #include "..\Minecraft.World\net.minecraft.network.h"
+#include "Windows64\Windows64_NameXuid.h"
 #include "..\Minecraft.World\Pos.h"
 #include "..\Minecraft.World\ProgressListener.h"
 #include "..\Minecraft.World\HellRandomLevelSource.h"
@@ -520,12 +521,13 @@ shared_ptr<ServerPlayer> PlayerList::getPlayerForLogin(PendingConnection *pendin
 	player->setOnlineXuid( onlineXuid ); // 4J Added
 #ifdef _WINDOWS64
 	{
+		PlayerUID persistentXuid = Win64NameXuid::ResolvePersistentXuidFromName(userName);
+		player->setXuid(persistentXuid);
+
 		INetworkPlayer* np = pendingConnection->connection->getSocket()->getPlayer();
 		if (np != NULL)
 		{
-			PlayerUID realXuid = np->GetUID();
-			player->setXuid(realXuid);
-			player->setOnlineXuid(realXuid);
+			player->setOnlineXuid(np->GetUID());
 		}
 	}
 #endif