feat(io): MusicController + MusicBackend (testable music logic)

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

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

@@ -0,0 +1,17 @@
+package thb.jeanluc.adventure.io;
+
+/** Raw audio operations — the only part that touches sound hardware. */
+public interface MusicBackend {
+
+    /** Fade the current track out, then start {@code track} (looping) and fade in to {@code gainDb}. */
+    void fadeTo(String track, float gainDb);
+
+    /** Fade the current track out and stop. */
+    void fadeOut();
+
+    /** Set the playback volume immediately. */
+    void setGainDb(float gainDb);
+
+    /** Stop playback and release resources. */
+    void shutdown();
+}

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

@@ -0,0 +1,52 @@
+package thb.jeanluc.adventure.io;
+
+import java.util.Objects;
+
+/**
+ * Frontend-agnostic music decision logic. Idempotent per track (never restarts
+ * the same track), fades to silence on a null/blank track, and honours the
+ * {@link MusicLevel} (OFF disables playback). Delegates raw operations to a
+ * {@link MusicBackend}.
+ */
+public final class MusicController {
+
+    private final MusicBackend backend;
+    private MusicLevel level = MusicLevel.MEDIUM;
+    private String current; // currently selected track, or null = silence
+
+    public MusicController(MusicBackend backend) {
+        this.backend = backend;
+    }
+
+    /** Called per turn with the current room's music field (may be null/blank). */
+    public void room(String track) {
+        if (!level.isOn()) {
+            return;
+        }
+        String next = (track == null || track.isBlank()) ? null : track;
+        if (Objects.equals(next, current)) {
+            return;
+        }
+        current = next;
+        if (next == null) {
+            backend.fadeOut();
+        } else {
+            backend.fadeTo(next, level.gainDb());
+        }
+    }
+
+    /** Applies a new music level (volume / off). */
+    public void level(MusicLevel newLevel) {
+        this.level = newLevel;
+        if (!newLevel.isOn()) {
+            backend.fadeOut();
+            current = null; // turning back on replays on the next room() call
+        } else {
+            backend.setGainDb(newLevel.gainDb());
+        }
+    }
+
+    public void shutdown() {
+        backend.shutdown();
+    }
+}