Player stats mechanics, level scaling, HUD polish
- Move crosshair target to Player from Gameplay
- Add level label to HUD, centered on stats icon
- Replace static min/max/val on RadialProgress with callbacks
- Wire gauge callbacks to Player stat getters
- Add stat decay over time: hunger, thirst, energy
- Add level-scaled max stat formulas (exponential for HP/mana/XP)
- Add getNextLevelXp with exponential level curve
- Fix Guardian applyDamage: play death immediately if HP hits zero
- Remove unused hp-ico and mana-ico assets
- Add target-ico intermediate blend asset
- Update player default stats to full in gameplay.json
- Change xp field from int64 to double