RPG Attribute System

Fixes

• Fixed multiplayer attribute desync caused by @p command selector targeting nearest player instead of source player
• Fixed players receiving incorrect max health and other attributes when joining near another player
• Fixed attribute sync missing after respawn on Fabric (UI showed Level 0 after death)
• Fixed stale attribute entries persisting in PlayerVariables after sync and clone operations
• Fixed NeoForge double-copy on death causing undefined clone behavior

Improvements

• Improved cross-loader attribute synchronization consistency (Fabric + NeoForge)
• Improved thread safety in AttributeManager for LAN/integrated server environments