feat: couple terrain surge and sun bloom to the soundtrack's low end

tests/unit/music-pulse.test.ts

@@ -0,0 +1,28 @@
+import { afterEach, describe, expect, it } from 'vitest';
+import { attachMusicPulse, readMusicPulse, setMusicPulseAudible } from '@/audio/music-pulse';
+
+const fakeAnalyser = (level: number) => ({
+  frequencyBinCount: 8,
+  getByteFrequencyData(target: Uint8Array) { target.fill(level); },
+}) as unknown as AnalyserNode;
+
+afterEach(() => {
+  attachMusicPulse(null);
+  setMusicPulseAudible(false);
+});
+
+describe('music pulse', () => {
+  it('reads zero without an analyser', () => {
+    setMusicPulseAudible(true);
+    expect(readMusicPulse()).toBe(0);
+  });
+  it('reads zero while the soundtrack is not audible', () => {
+    attachMusicPulse(fakeAnalyser(255));
+    expect(readMusicPulse()).toBe(0);
+  });
+  it('normalizes the low-band level when audible', () => {
+    attachMusicPulse(fakeAnalyser(51));
+    setMusicPulseAudible(true);
+    expect(readMusicPulse()).toBeCloseTo(0.2);
+  });
+});