WIP moving difficulty selection to engine

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

packages/wolf_3d_flutter/lib/wolf_3d_flutter.dart

@@ -21,6 +21,12 @@ class Wolf3d {
   /// Shared engine audio backend used by menus and gameplay sessions.
   final EngineAudio audio = WolfAudio();
 
+  /// Engine menu background color as 24-bit RGB.
+  int menuBackgroundRgb = 0x890000;
+
+  /// Engine menu panel color as 24-bit RGB.
+  int menuPanelRgb = 0x590002;
+
   /// Shared Flutter input adapter reused by gameplay screens.
   final Wolf3dFlutterInput input = Wolf3dFlutterInput();
 
@@ -41,6 +47,54 @@ class Wolf3d {
   /// Index of the episode currently selected in the UI flow.
   int get activeEpisode => _activeEpisode;
 
+  Difficulty? _activeDifficulty;
+
+  /// The difficulty applied when [launchEngine] creates a new session.
+  Difficulty? get activeDifficulty => _activeDifficulty;
+
+  /// Stores [difficulty] so the next [launchEngine] call uses it.
+  void setActiveDifficulty(Difficulty difficulty) {
+    _activeDifficulty = difficulty;
+  }
+
+  /// Clears any previously selected difficulty so the engine can prompt for one.
+  void clearActiveDifficulty() {
+    _activeDifficulty = null;
+  }
+
+  WolfEngine? _engine;
+
+  /// The most recently launched engine.
+  ///
+  /// Throws a [StateError] until [launchEngine] has been called.
+  WolfEngine get engine {
+    if (_engine == null) {
+      throw StateError('No engine launched. Call launchEngine() first.');
+    }
+    return _engine!;
+  }
+
+  /// Creates and initializes a [WolfEngine] for the current session config.
+  ///
+  /// Uses [activeGame], [activeEpisode], and [activeDifficulty]. Stores the
+  /// engine so it can be retrieved via [engine]. [onGameWon] is invoked when
+  /// the player completes the final level of the episode.
+  WolfEngine launchEngine({required void Function() onGameWon}) {
+    _engine = WolfEngine(
+      data: activeGame,
+      difficulty: _activeDifficulty,
+      startingEpisode: _activeEpisode,
+      frameBuffer: FrameBuffer(320, 200),
+      menuBackgroundRgb: menuBackgroundRgb,
+      menuPanelRgb: menuPanelRgb,
+      audio: audio,
+      input: input,
+      onGameWon: onGameWon,
+    );
+    _engine!.init();
+    return _engine!;
+  }
+
   /// Sets the active episode for the current [activeGame].
   void setActiveEpisode(int episodeIndex) {
     if (_activeGame == null) {