feat: Implement audio shutdown procedure for graceful app exit

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

packages/wolf_3d_flutter/lib/wolf_3d_flutter.dart

@@ -26,6 +26,8 @@ class Wolf3d {
   /// Shared engine audio backend used by menus and gameplay sessions.
   final EngineAudio audio;
 
+  Future<void>? _audioShutdownFuture;
+
   /// Engine menu background color as 24-bit RGB.
   int menuBackgroundRgb = 0x890000;
 
@@ -246,6 +248,24 @@ class Wolf3d {
     return this;
   }
 
+  /// Stops and disposes shared audio exactly once for app shutdown.
+  ///
+  /// Repeated calls return the same in-flight/completed future so hosts can
+  /// safely invoke shutdown from multiple lifecycle paths.
+  Future<void> shutdownAudio() {
+    final existing = _audioShutdownFuture;
+    if (existing != null) {
+      return existing;
+    }
+
+    final shutdown = () async {
+      await audio.stopAllAudio();
+      audio.dispose();
+    }();
+    _audioShutdownFuture = shutdown;
+    return shutdown;
+  }
+
   /// Loads an asset from the Flutter bundle, returning `null` when absent.
   Future<ByteData?> _tryLoad(String path) async {
     try {