Move level count to getter

Signed-off-by: Hans Kokx <hans.d.kokx@gmail.com>

packages/wolf_3d_data_types/lib/src/wolfenstein_data.dart

@@ -10,6 +10,11 @@ class WolfensteinData {
   final List<WolfLevel> levels;
   final List<VgaImage> vgaImages;
 
+  // --- Derived Properties ---
+  /// Calculates the number of available episodes based on the loaded levels.
+  /// (Each episode consists of exactly 10 levels).
+  int get numberOfEpisodes => (levels.length / 10).floor().clamp(1, 6);
+
   const WolfensteinData({
     required this.version,
     required this.walls,