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/ClientConnection.cpp

@@ -762,6 +762,18 @@ void ClientConnection::handleAddPlayer(shared_ptr<AddPlayerPacket> packet)
 			return;
 		}
 	}
+#ifdef _WINDOWS64
+	// Win64 player-data XUIDs are resolved from player name, so also guard against creating
+	// a duplicate remote player for a local slot by checking the username directly.
+	for (unsigned int idx = 0; idx < XUSER_MAX_COUNT; ++idx)
+	{
+		if (minecraft->localplayers[idx] != NULL && minecraft->localplayers[idx]->name == packet->name)
+		{
+			app.DebugPrintf("AddPlayerPacket received for local player name %ls\n", packet->name.c_str());
+			return;
+		}
+	}
+#endif
 /*#ifdef _WINDOWS64
 	// On Windows64 all XUIDs are INVALID_XUID so the XUID check above never fires.
 	// packet->m_playerIndex is the server-assigned sequential index (set via LoginPacket),
@@ -3957,4 +3969,4 @@ ClientConnection::DeferredEntityLinkPacket::DeferredEntityLinkPacket(shared_ptr<
 {
 	m_recievedTick = GetTickCount();
 	m_packet = packet;
-}
+}