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/terrain.ts

@@ -24,6 +24,8 @@ export interface TerrainWorld {
     overdrive?: number,
     surge?: number,
     spectrumPhase?: number,
+    /** 0 driving, 1 parked: the grid fades out under the end shot. */
+    engineOff?: number,
   ): void;
 }
 
@@ -59,6 +61,7 @@ export function createTerrain(
     uSurge: { value: 0 },
     uSpectrumPhase: { value: 0 },
     uRouteProgress: { value: 0 },
+    uEngineOff: { value: 0 },
     uTerrainEnergy: { value: initialPalette.terrainEnergy },
     uOctaves: { value: quality.terrainOctaves },
     uRelief: { value: 49 },
@@ -180,6 +183,7 @@ export function createTerrain(
       uniform vec3 uFogColor;
       uniform vec3 uHazeColor;
       uniform float uRouteProgress;
+      uniform float uEngineOff;
       uniform float uOverdrive;
       uniform float uSurge;
       varying float vHeight;
@@ -200,7 +204,8 @@ export function createTerrain(
         float road = 1.0 - smoothstep(7.6, 8.4, abs(vTerrainPosition.x));
         float apron = 1.0 - smoothstep(12.0, 32.0, abs(vTerrainPosition.x));
         float surfaceStrength = mix(0.72, 0.12 + uOverdrive * 0.3, road);
-        float alpha = vDepthFade * pulse * routeCalm * mix(surfaceStrength, 0.91, ridgeMix);
+        float alpha = vDepthFade * pulse * routeCalm * mix(surfaceStrength, 0.91, ridgeMix)
+          * (1.0 - uEngineOff * 0.72);
         float charge = uOverdrive * (0.35 + vFlow * 0.8) * (0.5 + apron * 0.5);
         charge += uSurge * (0.28 + vCrest * 0.5) * (1.0 - road);
         gl_FragColor = vec4(color * (1.46 + ridgeMix * 0.54 + charge), alpha);
@@ -282,7 +287,9 @@ export function createTerrain(
       overdrive = 0,
       surge = 0,
       spectrumPhase = 0,
+      engineOff = 0,
     ) {
+      uniforms.uEngineOff.value = clamp01(engineOff);
       const chapterPalette = CHAPTER_PALETTES[chapter];
       uniforms.uDistance.value =
         ((finiteOrZero(distance) % NOISE_PERIOD) + NOISE_PERIOD) % NOISE_PERIOD;