fix: null guard for unassigned LevelDefinition

Assets/Scripts/Infrastructure/Unity/GameBootstrap.cs

@@ -133,8 +133,11 @@ namespace Infrastructure.Unity
                 // Note: Generator uses negative offsets: 0, -15, -30.
                 // So Floor 0 is at Y=0. Floor 1 is at Y=-15.
 
-                var heightDist = levelGenerator.Definition.FloorHeightDistance;
-                var maxFloors = levelGenerator.Definition.FloorCount;
+                var def = levelGenerator.Definition;
+                if (def == null) return;
+
+                var heightDist = def.FloorHeightDistance;
+                var maxFloors = def.FloorCount;
     
                 var rawFloor = Mathf.RoundToInt(-playerY / heightDist);
                 _currentPlayerFloorIndex = Mathf.Clamp(rawFloor, 0, maxFloors - 1);

Assets/Scripts/Infrastructure/Unity/LevelGenerator.cs

@@ -30,6 +30,13 @@ namespace Infrastructure.Unity
         
         public IEnumerator GenerateAsync(SoundManager soundManager, TileRegistry registry, CameraController camera, RumbleManager rumble, Action onComplete)
         {
+            if (levelDefinition == null)
+            {
+                Debug.LogError("LevelGenerator: levelDefinition is not assigned. Assign a LevelDefinition asset in the Inspector.");
+                onComplete?.Invoke();
+                yield break;
+            }
+
             _tilePool = new TilePool(tilePrefab, transform);
 
             var stopwatch = new Stopwatch();