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).

Minecraft.Client/Windows64/Windows64_NameXuid.h

@@ -0,0 +1,46 @@
+#pragma once
+
+#ifdef _WINDOWS64
+
+#include <string>
+
+namespace Win64NameXuid
+{
+	/**
+	 * ## Resolves a persistent 64-bit player ID from the player's username.
+	 * 
+	 * We keep this deterministic so existing player save/map systems can key off XUID.
+	 * 
+	 * @param playerName The player's username.
+	 * @return The resolved PlayerUID.
+	 */
+	inline PlayerUID ResolvePersistentXuidFromName(const std::wstring &playerName)
+	{
+		const unsigned __int64 fnvOffset = 14695981039346656037ULL;
+		const unsigned __int64 fnvPrime = 1099511628211ULL;
+		unsigned __int64 hash = fnvOffset;
+
+		for (size_t i = 0; i < playerName.length(); ++i)
+		{
+			unsigned short codeUnit = (unsigned short)playerName[i];
+			hash ^= (unsigned __int64)(codeUnit & 0xFF);
+			hash *= fnvPrime;
+			hash ^= (unsigned __int64)((codeUnit >> 8) & 0xFF);
+			hash *= fnvPrime;
+		}
+
+		// Namespace the hash away from legacy smallId-based values.
+		hash ^= 0x9E3779B97F4A7C15ULL;
+		hash |= 0x8000000000000000ULL;
+
+		if (hash == (unsigned __int64)INVALID_XUID)
+		{
+			hash ^= 0x0100000000000001ULL;
+		}
+
+		return (PlayerUID)hash;
+	}
+}
+
+#endif
+