fix(io): idempotent MusicController.level; assert gain forwarding + shutdown

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Semesterprojekt/src/main/java/thb/jeanluc/adventure/io/MusicController.java

@@ -37,6 +37,9 @@ public final class MusicController {
 
     /** Applies a new music level (volume / off). */
     public void level(MusicLevel newLevel) {
+        if (newLevel == this.level) {
+            return;
+        }
         this.level = newLevel;
         if (!newLevel.isOn()) {
             backend.fadeOut();