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();

Semesterprojekt/src/test/java/thb/jeanluc/adventure/io/MusicControllerTest.java

@@ -12,7 +12,8 @@ class MusicControllerTest {
     /** Records backend calls for assertions. */
     private static final class FakeBackend implements MusicBackend {
         final List<String> calls = new ArrayList<>();
-        public void fadeTo(String track, float gainDb) { calls.add("fadeTo:" + track); }
+        float lastGain = Float.NaN;
+        public void fadeTo(String track, float gainDb) { calls.add("fadeTo:" + track); lastGain = gainDb; }
         public void fadeOut() { calls.add("fadeOut"); }
         public void setGainDb(float gainDb) { calls.add("setGain:" + gainDb); }
         public void shutdown() { calls.add("shutdown"); }
@@ -84,4 +85,22 @@ class MusicControllerTest {
         c.room("a");               // current was reset → plays again
         assertThat(b.calls).containsExactly("fadeTo:a");
     }
+
+    @Test
+    void forwardsTheCurrentLevelGain() {
+        FakeBackend b = new FakeBackend();
+        MusicController c = new MusicController(b);
+        c.room("a");
+        assertThat(b.lastGain).isEqualTo(MusicLevel.MEDIUM.gainDb());
+        c.level(MusicLevel.HIGH);
+        c.room("b");
+        assertThat(b.lastGain).isEqualTo(MusicLevel.HIGH.gainDb());
+    }
+
+    @Test
+    void shutdownDelegatesToBackend() {
+        FakeBackend b = new FakeBackend();
+        new MusicController(b).shutdown();
+        assertThat(b.calls).containsExactly("shutdown");
+    }
 }