fix: rebalance procedural audio mix

tests/unit/audio-lifecycle.test.ts

@@ -212,6 +212,29 @@ describe('audio controller lifecycle', () => {
 });
 
 describe('procedural audio graph', () => {
+  it('keeps the soundtrack clearly ahead of the ambient drive effects across intensity changes', async () => {
+    vi.useFakeTimers();
+    const context = new FakeAudioContext();
+
+    try {
+      const graph = createSynthEngine(() => context as unknown as AudioContext);
+      const [, musicGain, engineGain, , , roadGain, ignitionGain] = context.gains;
+
+      expect(musicGain?.gain.value).toBe(0.78);
+      expect(engineGain?.gain.value).toBe(0.09);
+      expect(roadGain?.gain.value).toBe(0.018);
+      expect(ignitionGain?.gain.scheduledValues).toEqual([0.0001, 0.025, 0.0001]);
+
+      graph.setIntensity(1);
+      expect(engineGain?.gain.targetValues).toContain(0.13);
+      expect(roadGain?.gain.targetValues).toContain(0.038);
+
+      await graph.close();
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+
   it('routes independent music, engine, and road buses through compression before the master output', async () => {
     vi.useFakeTimers();
     const context = new FakeAudioContext();