[2pts] Implement player leveling and stat progression

[github-actions]

Description

Implement player leveling system with XP thresholds and stat progression.

Depends on: Player class issue

Level System

• Define XP thresholds for levels 1-10
• Example: Level 2 = 100 XP, Level 3 = 250 XP, etc.

Level Up Effects

When player levels up:

• Increment Level property
• Increase AttackPower (e.g., +2)
• Increase Defense (e.g., +1)
• Increase MaxHealth (e.g., +10)
• Restore some Health (e.g., 50% of MaxHealth)
• Publish PlayerLevelUpEvent via GameEventSystem

Progress Tracking

• GetProgressToNextLevel() method
• Returns double (0.0 - 1.0) representing progress
• Used for UI progress bars

Acceptance Criteria

• XP thresholds defined for levels 1-10
• Level up logic implemented in Player.AddExperience()
• AttackPower increases on level up
• Defense increases on level up
• MaxHealth increases on level up
• Health restored partially on level up
• PlayerLevelUpEvent published via GameEventSystem
• GetProgressToNextLevel() method implemented
• Returns 0.0 - 1.0 progress value
• Method documented with XML comments