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

Minecraft.Client/Common/Network/PlatformNetworkManagerStub.cpp

@@ -5,6 +5,7 @@
 #include "..\..\Xbox\Network\NetworkPlayerXbox.h"
 #ifdef _WINDOWS64
 #include "..\..\Windows64\Network\WinsockNetLayer.h"
+#include "..\..\Windows64\Windows64_Xuid.h"
 #include "..\..\Minecraft.h"
 #include "..\..\User.h"
 #include <iostream>
@@ -234,6 +235,7 @@ void CPlatformNetworkManagerStub::DoWork()
 				qnetPlayer->m_smallId = 0;
 				qnetPlayer->m_isRemote = false;
 				qnetPlayer->m_isHostPlayer = false;
+				qnetPlayer->m_resolvedXuid = INVALID_XUID;
 				qnetPlayer->m_gamertag[0] = 0;
 				qnetPlayer->SetCustomDataValue(0);
 				WinsockNetLayer::PushFreeSmallId(disconnectedSmallId);
@@ -354,7 +356,9 @@ void CPlatformNetworkManagerStub::HostGame(int localUsersMask, bool bOnlineGame,
 #ifdef _WINDOWS64
 	IQNet::m_player[0].m_smallId = 0;
 	IQNet::m_player[0].m_isRemote = false;
+	// world host is pinned to legacy host XUID to keep old player data compatibility.
 	IQNet::m_player[0].m_isHostPlayer = true;
+	IQNet::m_player[0].m_resolvedXuid = Win64Xuid::GetLegacyEmbeddedHostXuid();
 	IQNet::s_playerCount = 1;
 #endif
 
@@ -411,6 +415,8 @@ int CPlatformNetworkManagerStub::JoinGame(FriendSessionInfo* searchResult, int l
 	IQNet::m_player[0].m_smallId = 0;
 	IQNet::m_player[0].m_isRemote = true;
 	IQNet::m_player[0].m_isHostPlayer = true;
+	// Remote host still maps to legacy host XUID in mixed old/new sessions.
+	IQNet::m_player[0].m_resolvedXuid = Win64Xuid::GetLegacyEmbeddedHostXuid();
 	wcsncpy_s(IQNet::m_player[0].m_gamertag, 32, searchResult->data.hostName, _TRUNCATE);
 
 	WinsockNetLayer::StopDiscovery();
@@ -426,6 +432,8 @@ int CPlatformNetworkManagerStub::JoinGame(FriendSessionInfo* searchResult, int l
 	IQNet::m_player[localSmallId].m_smallId = localSmallId;
 	IQNet::m_player[localSmallId].m_isRemote = false;
 	IQNet::m_player[localSmallId].m_isHostPlayer = false;
+	// Local non-host identity is the persistent uid.dat XUID.
+	IQNet::m_player[localSmallId].m_resolvedXuid = Win64Xuid::ResolvePersistentXuid();
 
 	Minecraft* pMinecraft = Minecraft::GetInstance();
 	wcscpy_s(IQNet::m_player[localSmallId].m_gamertag, 32, pMinecraft->user->name.c_str());