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/Windows64/Windows64_Minecraft.cpp

@@ -42,6 +42,7 @@
 #include "..\..\Minecraft.World\OldChunkStorage.h"
 #include "Common/PostProcesser.h"
 #include "Network\WinsockNetLayer.h"
+#include "Windows64_Xuid.h"
 
 #include "Xbox/resource.h"
 
@@ -1221,6 +1222,12 @@ int APIENTRY _tWinMain(_In_ HINSTANCE hInstance,
 	Win64LaunchOptions launchOptions = ParseLaunchOptions();
 	ApplyScreenMode(launchOptions.screenMode);
 
+	// Ensure uid.dat exists from startup in client mode (before any multiplayer/login path).
+	if (!launchOptions.serverMode)
+	{
+		Win64Xuid::ResolvePersistentXuid();
+	}
+
 	// If no username, let's fall back
 	if (g_Win64Username[0] == 0)
 	{