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/audio/drive-texture.ts

@@ -10,11 +10,15 @@ const CRUISE_ROAD_GAIN = 0.006;
 const MAX_ROAD_GAIN = 0.024;
 const PASS_BY_GAIN = 0.05;
 const PASS_BY_SECONDS = 1.6;
+/** Engine off at the destination: a low-pass sweep down to a parked idle. */
+const PARK_SECONDS = 1.2;
 
 export interface DriveTexture {
   setEnergy(sample: DriveEnergySample): void;
   /** Passing under the gantry: a filtered air sweep across the stereo field. */
   passBy?(): void;
+  /** Parked at the final exit (true) or driving again (false). */
+  park?(parked: boolean): void;
   dispose(): void;
 }
 
@@ -141,6 +145,7 @@ export const createDriveTexture: DriveTextureFactory = (context, destination) =>
 
   let currentEnergy = 0;
   let disposed = false;
+  let parked = false;
   const automate = (parameter: AudioParam, value: number, duration: number): void => {
     const now = context.currentTime;
     holdParameter(parameter, now);
@@ -151,6 +156,10 @@ export const createDriveTexture: DriveTextureFactory = (context, destination) =>
     setEnergy(sample) {
       if (disposed) return;
       const next = Math.min(1, Math.max(0, sample.energy));
+      if (parked) {
+        currentEnergy = next;
+        return;
+      }
       const duration = next > currentEnergy ? ATTACK_SECONDS : RELEASE_SECONDS;
       automate(fundamental.source.frequency, 42 + next * 20, duration);
       automate(harmonic.source.frequency, 84 + next * 40, duration);
@@ -160,6 +169,24 @@ export const createDriveTexture: DriveTextureFactory = (context, destination) =>
       automate(roadGain.gain, CRUISE_ROAD_GAIN + next * (MAX_ROAD_GAIN - CRUISE_ROAD_GAIN), duration);
       currentEnergy = next;
     },
+    park(next) {
+      if (disposed || next === parked) return;
+      parked = next;
+      if (!parked) {
+        const energy = currentEnergy;
+        currentEnergy = -1;
+        this.setEnergy({ energy, source: 'idle' });
+        return;
+      }
+      const now = context.currentTime;
+      for (const parameter of [engineFilter.frequency, roadLowpass.frequency, engineGain.gain, roadGain.gain]) {
+        holdParameter(parameter, now);
+      }
+      engineFilter.frequency.exponentialRampToValueAtTime(70, now + PARK_SECONDS);
+      roadLowpass.frequency.exponentialRampToValueAtTime(260, now + PARK_SECONDS);
+      engineGain.gain.linearRampToValueAtTime(CRUISE_ENGINE_GAIN * 0.3, now + PARK_SECONDS);
+      roadGain.gain.linearRampToValueAtTime(CRUISE_ROAD_GAIN * 0.25, now + PARK_SECONDS);
+    },
     passBy() {
       if (disposed) return;
       const now = context.currentTime;