refactor: fix bugs and improve architecture

- Fix double-execution bug in LevelStateHandler (coins/skills were committed twice per level)
- Fix DamageComponent to track multiple targets via HashSet instead of single node
- Fix HealthComponent: update health immediately, decouple from PlayerController via signal wiring in PlayerController
- Remove dead loop in SkillManager.RemoveSkill; simplify O(n²) throw skill loop
- Replace GetNode<T>(path) with .Instance across managers; add Instance to StatisticsManager/SpeedRunManager
- GameManager.GetPlayer() now uses EventBus.PlayerSpawned instead of scanning all scene nodes
- UIManager.UiStack: remove [Export], use private List<Control>
- PlayerState: extract DefaultLives constant, simplify CreateDefault()

scripts/Events/LevelStateHandler.cs

@@ -23,17 +23,7 @@ public partial class LevelStateHandler : Node
 
     private void OnLevelCompleted(int levelIndex, Node currentScene, double completionTime)
     {
-        var store = GameStateStore.Instance;
-        if (store == null) return;
-
-        // Mark level complete and unlock next
-        store.MarkLevelComplete(levelIndex);
-
-        // Commit session data to persistent state
-        store.CommitSessionCoins();
-        store.CommitSessionSkills();
-
-        // Reset session for next level
-        store.ResetSession();
+        // State mutations (commit coins/skills, reset session) are handled by GameManager.OnLevelComplete
+        // before this event fires. This handler is reserved for future level-specific side-effects.
     }
 }