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

src/scene/world/environment.ts

@@ -1,5 +1,6 @@
 import * as THREE from 'three';
 import type { JourneySnapshot } from '@/journey/types';
+import { readMusicPulse } from '@/audio/music-pulse';
 import type { SceneResourceBag } from '@/scene/resources';
 import type { SceneBuildContext, SceneGeneration } from '@/scene/types';
 import { renderQualityFor } from '@/scene/render-quality';
@@ -165,6 +166,7 @@ export async function createProceduralSceneGeneration(
   let signCameraWeight = 0;
   let overdrive = 0;
   let terrainSurge = 0;
+  let musicPulse = 0;
   let terrainSpectrumPhase = 0;
   let firstFrameSuppressionRecorded = false;
 
@@ -264,10 +266,17 @@ export async function createProceduralSceneGeneration(
       const surgeTarget = Math.sqrt(
         THREE.MathUtils.clamp(snapshot.acceleration, 0, 1),
       );
+      // Soundtrack coupling: the bass the visitor hears lifts the terrain and
+      // the sun. One analyser read per frame; zero when nothing is audible.
+      const pulseTarget = readMusicPulse();
+      musicPulse +=
+        (pulseTarget - musicPulse) *
+        (1 - Math.exp(-deltaSeconds / (pulseTarget > musicPulse ? 0.05 : 0.32)));
+      const beat = THREE.MathUtils.smoothstep(musicPulse, 0.42, 0.92);
       terrainSurge +=
-        (surgeTarget - terrainSurge) *
+        (Math.max(surgeTarget, beat * 0.55) - terrainSurge) *
         (1 -
-          Math.exp(-deltaSeconds / (surgeTarget > terrainSurge ? 0.11 : 1.15)));
+          Math.exp(-deltaSeconds / (Math.max(surgeTarget, beat * 0.55) > terrainSurge ? 0.11 : 1.15)));
       // Integrate color travel instead of multiplying elapsed time by speed:
       // lifting off slows the spectrum to rest without rewinding its phase.
       terrainSpectrumPhase =
@@ -314,7 +323,7 @@ export async function createProceduralSceneGeneration(
         currentPalette,
         snapshot.routeProgress,
         elapsedSeconds,
-        response.bloomEnergy,
+        Math.min(1, response.bloomEnergy + beat * 0.35),
         snapshot.arrived,
       );
       car.update(