feat: engine off at the destination

On arrival the drive winds down to a stop, the tail and running lamps
drop to parking level, the sky darkens about 10% with more stars and
the grid fades. The engine and road noise take a 1.2 s low-pass sweep
to idle. The letterbox stays within 8vh and the closing credit types
into the lower bar; reduced motion keeps only the bars and dimming.

src/scene/world/environment.ts

@@ -307,9 +307,17 @@ export async function createProceduralSceneGeneration(
       terrainSpectrumPhase =
         (terrainSpectrumPhase + deltaSeconds * overdrive * 0.38) % 1;
       writeChapterPalette(targetPalette, snapshot.chapter, snapshot.chapter, 0);
+      // Engine off: parked at the destination the sky drops about 10%, more
+      // stars come out, and the drive winds down to a stop.
+      const engineOff = THREE.MathUtils.smoothstep(outroProgress, 0.12, 0.7);
+      if (engineOff > 0) {
+        targetPalette.sky.lerp(ENGINE_OFF_NIGHT, 0.1 * engineOff);
+        targetPalette.fog.lerp(ENGINE_OFF_NIGHT, 0.1 * engineOff);
+        targetPalette.starVisibility *= 1 + 0.8 * engineOff;
+      }
       dampPalette(currentPalette, targetPalette, deltaSeconds);
       const driveDistance = travelDistance.advance(
-        deltaSeconds,
+        deltaSeconds * (1 - engineOff),
         snapshot.driveSpeed,
         response.roadEnergy,
       );
@@ -317,7 +325,7 @@ export async function createProceduralSceneGeneration(
         0.18 + response.roadEnergy * 0.82,
         0.18,
         1,
-      );
+      ) * (1 - engineOff);
       road.update(
         driveDistance,
         visualSpeed,
@@ -347,6 +355,7 @@ export async function createProceduralSceneGeneration(
         overdrive,
         terrainSurge,
         terrainSpectrumPhase,
+        engineOff,
       );
       atmosphericTime += deltaSeconds;
       const elapsedSeconds = atmosphericTime;
@@ -354,7 +363,7 @@ export async function createProceduralSceneGeneration(
       skyline.update(currentPalette, elapsedSeconds);
       traffic.update(
         deltaSeconds,
-        18 + snapshot.driveSpeed * 54 + response.roadEnergy * 48,
+        (18 + snapshot.driveSpeed * 54 + response.roadEnergy * 48) * (1 - engineOff),
         elapsedSeconds,
       );
       sun.update(
@@ -369,6 +378,7 @@ export async function createProceduralSceneGeneration(
         response.suspensionEnergy,
         elapsedSeconds,
         driveDistance,
+        engineOff,
       );
       speedWake.update(
         overdrive,
@@ -634,6 +644,7 @@ const INTRO_SECONDS = 4.2;
 const OUTRO_SECONDS = 5.5;
 const OUTRO_RETURN_SECONDS = 1.6;
 const OUTRO_REACH = 0.62;
+const ENGINE_OFF_NIGHT = new THREE.Color(0x000000);
 /** Plates rise back over 720 ms with up to three 90 ms staggers. */
 const INTERLUDE_RISE_MS = 1100;