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/Extrax64Stubs.cpp

@@ -21,6 +21,7 @@
 #include "Windows64\Social\SocialManager.h"
 #include "Windows64\Sentient\DynamicConfigurations.h"
 #include "Windows64\Network\WinsockNetLayer.h"
+#include "Windows64\Windows64_Xuid.h"
 #elif defined __PSVITA__
 #include "PSVita\Sentient\SentientManager.h"
 #include "StatsCounter.h"
@@ -200,7 +201,15 @@ DWORD IQNetPlayer::GetCurrentRtt() { return 0; }
 bool IQNetPlayer::IsHost() { return m_isHostPlayer; }
 bool IQNetPlayer::IsGuest() { return false; }
 bool IQNetPlayer::IsLocal() { return !m_isRemote; }
-PlayerUID IQNetPlayer::GetXuid() { return (PlayerUID)(0xe000d45248242f2e + m_smallId); } // todo: restore to INVALID_XUID once saves support this
+PlayerUID IQNetPlayer::GetXuid()
+{
+	// Compatibility model:
+	// - Preferred path: use per-player resolved XUID populated from login/add-player flow.
+	// - Fallback path: keep legacy base+smallId behavior for peers/saves still on old scheme.
+	if (m_resolvedXuid != INVALID_XUID)
+		return m_resolvedXuid;
+	return (PlayerUID)(0xe000d45248242f2e + m_smallId);
+}
 LPCWSTR IQNetPlayer::GetGamertag() { return m_gamertag; }
 int IQNetPlayer::GetSessionIndex() { return m_smallId; }
 bool IQNetPlayer::IsTalking() { return false; }
@@ -226,6 +235,7 @@ void Win64_SetupRemoteQNetPlayer(IQNetPlayer * player, BYTE smallId, bool isHost
 	player->m_smallId = smallId;
 	player->m_isRemote = !isLocal;
 	player->m_isHostPlayer = isHost;
+	player->m_resolvedXuid = INVALID_XUID;
 	swprintf_s(player->m_gamertag, 32, L"Player%d", smallId);
 	if (smallId >= IQNet::s_playerCount)
 		IQNet::s_playerCount = smallId + 1;
@@ -285,8 +295,13 @@ IQNetPlayer* IQNet::GetPlayerByXuid(PlayerUID xuid)
 {
 	for (DWORD i = 0; i < s_playerCount; i++)
 	{
-		if (Win64_IsActivePlayer(&m_player[i], i) && m_player[i].GetXuid() == xuid) return &m_player[i];
+		if (!Win64_IsActivePlayer(&m_player[i], i))
+			continue;
+
+		if (m_player[i].GetXuid() == xuid)
+			return &m_player[i];
 	}
+	// Keep existing stub behavior: return host slot instead of NULL on miss.
 	return &m_player[0];
 }
 DWORD IQNet::GetPlayerCount()
@@ -301,7 +316,13 @@ DWORD IQNet::GetPlayerCount()
 QNET_STATE IQNet::GetState() { return _iQNetStubState; }
 bool IQNet::IsHost() { return s_isHosting; }
 HRESULT IQNet::JoinGameFromInviteInfo(DWORD dwUserIndex, DWORD dwUserMask, const INVITE_INFO * pInviteInfo) { return S_OK; }
-void IQNet::HostGame() { _iQNetStubState = QNET_STATE_SESSION_STARTING; s_isHosting = true; }
+void IQNet::HostGame()
+{
+	_iQNetStubState = QNET_STATE_SESSION_STARTING;
+	s_isHosting = true;
+	// Host slot keeps legacy XUID so old host player data remains addressable.
+	m_player[0].m_resolvedXuid = Win64Xuid::GetLegacyEmbeddedHostXuid();
+}
 void IQNet::ClientJoinGame()
 {
 	_iQNetStubState = QNET_STATE_SESSION_STARTING;
@@ -312,6 +333,7 @@ void IQNet::ClientJoinGame()
 		m_player[i].m_smallId = (BYTE)i;
 		m_player[i].m_isRemote = true;
 		m_player[i].m_isHostPlayer = false;
+		m_player[i].m_resolvedXuid = INVALID_XUID;
 		m_player[i].m_gamertag[0] = 0;
 		m_player[i].SetCustomDataValue(0);
 	}
@@ -326,6 +348,7 @@ void IQNet::EndGame()
 		m_player[i].m_smallId = (BYTE)i;
 		m_player[i].m_isRemote = false;
 		m_player[i].m_isHostPlayer = false;
+		m_player[i].m_resolvedXuid = INVALID_XUID;
 		m_player[i].m_gamertag[0] = 0;
 		m_player[i].SetCustomDataValue(0);
 	}
@@ -575,10 +598,13 @@ void				C_4JProfile::GetXUID(int iPad, PlayerUID * pXuid, bool bOnlineXuid)
 		*pXuid = INVALID_XUID;
 		return;
 	}
+	// LoginPacket reads this value as client identity:
+	// - host keeps legacy host XUID for world compatibility
+	// - non-host uses persistent uid.dat-backed XUID
 	if (IQNet::s_isHosting)
-		*pXuid = 0xe000d45248242f2e;
+		*pXuid = Win64Xuid::GetLegacyEmbeddedHostXuid();
 	else
-		*pXuid = 0xe000d45248242f2e + WinsockNetLayer::GetLocalSmallId();
+		*pXuid = Win64Xuid::ResolvePersistentXuid();
 #else
 	* pXuid = 0xe000d45248242f2e + iPad;
 #endif