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/PendingConnection.cpp

@@ -161,6 +161,23 @@ void PendingConnection::handleLogin(shared_ptr<LoginPacket> packet)
 	//if (true)// 4J removed !server->onlineMode)
 	bool sentDisconnect = false;
 
+	// Use the same Xuid choice as handleAcceptedLogin (offline first, online fallback).
+	// 
+	PlayerUID loginXuid = packet->m_offlineXuid;
+	if (loginXuid == INVALID_XUID) loginXuid = packet->m_onlineXuid;
+
+	bool duplicateXuid = false;
+	if (loginXuid != INVALID_XUID && server->getPlayers()->getPlayer(loginXuid) != nullptr)
+	{
+		duplicateXuid = true;
+	}
+	else if (packet->m_onlineXuid != INVALID_XUID &&
+		packet->m_onlineXuid != loginXuid &&
+		server->getPlayers()->getPlayer(packet->m_onlineXuid) != nullptr)
+	{
+		duplicateXuid = true;
+	}
+
 	if( sentDisconnect )
 	{
 		// Do nothing
@@ -169,6 +186,12 @@ void PendingConnection::handleLogin(shared_ptr<LoginPacket> packet)
 	{
 		disconnect(DisconnectPacket::eDisconnect_Banned);
 	}
+	else if (duplicateXuid)
+	{
+		// if same XUID already in use by another player so disconnect this one.
+		app.DebugPrintf("Rejecting duplicate xuid for name: %ls\n", name.c_str());
+		disconnect(DisconnectPacket::eDisconnect_Banned);
+	}
 #ifdef _WINDOWS64
 	else if (g_bRejectDuplicateNames)
 	{