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).
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