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()

Autoloads/SpeedRunManager.cs

@@ -5,6 +5,11 @@ namespace Mr.BrickAdventures.Autoloads;
 
 public partial class SpeedRunManager : Node
 {
+    public static SpeedRunManager Instance { get; private set; }
+
+    public override void _EnterTree() => Instance = this;
+    public override void _ExitTree() { if (Instance == this) Instance = null; }
+
     public bool IsRunning { get; private set; } = false;
     public bool IsVisible { get; private set; } = false;