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

src/composables/useAudioEngine.ts

@@ -8,6 +8,7 @@ import {
 } from 'vue';
 import { createAudioController } from '@/audio/create-audio-controller';
 import { createBrowserSoundtrackEngine } from '@/audio/soundtrack-engine';
+import { setMusicPulseAudible } from '@/audio/music-pulse';
 import type {
   AudioController,
   AudioUiState,
@@ -65,6 +66,10 @@ export function useAudioEngine(sources: AudioEngineSources): AudioEngineBinding
   watch(sources.chapter, (intent) => controller?.setChapterIntent(intent), { flush: 'sync' });
   watch(sources.drive, (sample) => controller?.setDriveEnergy(sample), { flush: 'sync' });
   watch(sources.volume, (value) => controller?.setVolume(value));
+  // The world only answers the music the visitor can actually hear.
+  watch(state, (next) => {
+    setMusicPulseAudible(next.kind === 'running' && !next.muted && next.volume > 0);
+  });
 
   onMounted(() => {
     ensureController();