feat(io): MusicLevel enum (Off/Low/Med/High)

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

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

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+package thb.jeanluc.adventure.io;
+
+/** Background-music volume level, cycled in the Settings menu. */
+public enum MusicLevel {
+    OFF, LOW, MEDIUM, HIGH;
+
+    /** Target MASTER_GAIN in decibels. Irrelevant for {@link #OFF} (playback disabled). */
+    public float gainDb() {
+        return switch (this) {
+            case OFF -> Float.NEGATIVE_INFINITY;
+            case LOW -> -20f;
+            case MEDIUM -> -10f;
+            case HIGH -> 0f;
+        };
+    }
+
+    /** @return true when music should play. */
+    public boolean isOn() {
+        return this != OFF;
+    }
+
+    /** Next level in the cycle (wraps HIGH → OFF). */
+    public MusicLevel next() {
+        return values()[(ordinal() + 1) % values().length];
+    }
+}

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

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+package thb.jeanluc.adventure.io;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class MusicLevelTest {
+
+    @Test
+    void gainIncreasesWithLevel() {
+        assertThat(MusicLevel.LOW.gainDb()).isLessThan(MusicLevel.MEDIUM.gainDb());
+        assertThat(MusicLevel.MEDIUM.gainDb()).isLessThan(MusicLevel.HIGH.gainDb());
+    }
+
+    @Test
+    void isOnOnlyWhenNotOff() {
+        assertThat(MusicLevel.OFF.isOn()).isFalse();
+        assertThat(MusicLevel.LOW.isOn()).isTrue();
+        assertThat(MusicLevel.HIGH.isOn()).isTrue();
+    }
+
+    @Test
+    void nextCyclesThroughAllAndWraps() {
+        assertThat(MusicLevel.OFF.next()).isEqualTo(MusicLevel.LOW);
+        assertThat(MusicLevel.LOW.next()).isEqualTo(MusicLevel.MEDIUM);
+        assertThat(MusicLevel.MEDIUM.next()).isEqualTo(MusicLevel.HIGH);
+        assertThat(MusicLevel.HIGH.next()).isEqualTo(MusicLevel.OFF);
+    }
+}