Introduce uid.dat (offline PlayerUIDs), fix multiplayer save data persistence (#536)

* 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).

* fix: preserve legacy host xuid (base xuid + 0) for existing world compatibility

- Add legacy embedded host XUID helper (base + 0).
- When Minecraft.Client is hosting, force only the first host player to use legacy host XUID.
- Keep name-based XUID for non-host players.
- Prevent old singleplayer/hosted worlds from losing/mismatching host player data.

* update: migrate Win64 player uid to `uid.dat`-backed XUID and add XUID based duplicate login guards

- Replace Win64 username-derived XUID resolution with persistent `uid.dat`-backed identity (`Windows64_Xuid` / `Win64Xuid`).
- Persist a per-client XUID next to the executable, with first-run generation, read/write, and process-local caching.
- Keep legacy host compatibility by pinning host self to legacy embedded `base + 0` XUID for existing world/playerdata continuity.
- Propagate packet-authoritative XUIDs into QNet player slots via `m_resolvedXuid`, and use it for `GetXuid`/`GetPlayerByXuid` with legacy fallback.
- Update Win64 profile/network paths to use persistent XUID for non-host clients and clear resolved identity on disconnect.
- Add login-time duplicate checks: reject connections when the same XUID is already connected (in addition to existing duplicate-name checks on Win64).
- Add inline compatibility comments around legacy/new identity coexistence paths for easier future maintenance.

* update: ensure uid.dat exists at startup in client mode for multiplayer

1 files changed, 21 insertions, 5 deletions

diff --git a/Minecraft.Client/PlayerList.cpp b/Minecraft.Client/PlayerList.cpp
index c1ccfe14..42724a4f 100644
--- a/Minecraft.Client/PlayerList.cpp
+++ b/Minecraft.Client/PlayerList.cpp
@@ -18,6 +18,7 @@
 #include "..\Minecraft.World\ArrayWithLength.h"
 #include "..\Minecraft.World\net.minecraft.network.packet.h"
 #include "..\Minecraft.World\net.minecraft.network.h"
+#include "Windows64\Windows64_Xuid.h"
 #include "..\Minecraft.World\Pos.h"
 #include "..\Minecraft.World\ProgressListener.h"
 #include "..\Minecraft.World\HellRandomLevelSource.h"
@@ -106,11 +107,18 @@ void PlayerList::placeNewPlayer(Connection *connection, shared_ptr<ServerPlayer>
 	}
 #endif
 #ifdef _WINDOWS64
-	if (networkPlayer != NULL && !networkPlayer->IsLocal())
+	if (networkPlayer != NULL)
 	{
 		NetworkPlayerXbox* nxp = (NetworkPlayerXbox*)networkPlayer;
 		IQNetPlayer* qnp = nxp->GetQNetPlayer();
-		wcsncpy_s(qnp->m_gamertag, 32, player->name.c_str(), _TRUNCATE);
+		if (qnp != NULL)
+		{
+			if (!networkPlayer->IsLocal())
+			{
+				wcsncpy_s(qnp->m_gamertag, 32, player->name.c_str(), _TRUNCATE);
+			}
+			qnp->m_resolvedXuid = player->getXuid();
+		}
 	}
 #endif
 	// 4J Stu - TU-1 hotfix
@@ -520,12 +528,20 @@ shared_ptr<ServerPlayer> PlayerList::getPlayerForLogin(PendingConnection *pendin
 	player->setOnlineXuid( onlineXuid ); // 4J Added
 #ifdef _WINDOWS64
 	{
+		// Use packet-supplied identity from LoginPacket.
+		// Do not recompute from name here: mixed-version clients must stay compatible.
 		INetworkPlayer* np = pendingConnection->connection->getSocket()->getPlayer();
 		if (np != NULL)
 		{
-			PlayerUID realXuid = np->GetUID();
-			player->setXuid(realXuid);
-			player->setOnlineXuid(realXuid);
+			player->setOnlineXuid(np->GetUID());
+
+			// Backward compatibility: when Minecraft.Client is hosting, keep the first
+			// host player on the legacy embedded host XUID (base + 0).
+			// This preserves pre-migration host playerdata in existing worlds.
+			if (np->IsHost())
+			{
+				player->setXuid(Win64Xuid::GetLegacyEmbeddedHostXuid());
+			}
 		}
 	}
 #endif