feat: weave speed-reactive terrain and ground roadside signs

Anchor desktop signs beside the road and feather the compact reading edge with measured chapter landings. Add a continuous cyan-to-coral terrain grid and a bounded rear wake that builds with existing drive speed.

Preserve approved content, media, soundtrack, and canonical journey contracts.

src/scene/world/sign.ts

@@ -9,6 +9,7 @@ import {
   type SignViewportProfile,
 } from '@/journey/sign-routes';
 import type { SceneResourceBag } from '@/scene/resources';
+import { roadsideSignMetrics, usesRoadsideSign } from './sign-layout';
 
 interface ProjectingCamera {
   position: { x: number; y: number; z: number };
@@ -32,6 +33,7 @@ export interface SignWorld {
     height: number,
     navigationBottom: number,
     now: number,
+    carAnchor?: { x: number; z: number },
   ): void;
   setViewportAspect(aspect: number): void;
   setViewportSize(width: number, height: number): void;
@@ -407,6 +409,11 @@ export function createSign(
   let screenY = 120;
   let screenWidth = 340;
   let initialized = false;
+  let roadsideMix = 0;
+  let previousPhase: SignSnapshot['phase'] | null = null;
+  let dwellDrift = 0;
+  let departureDrift = 0;
+  const physical = roadsideSignMetrics(900);
   const point = new THREE.Vector3();
   const ray = new THREE.Vector3();
   const forward = new THREE.Vector3();
@@ -498,9 +505,13 @@ export function createSign(
       if (!sign) {
         group.visible = false;
         initialized = false;
+        previousPhase = null;
+        dwellDrift = 0;
         return;
       }
       if (target !== sign.targetId) {
+        previousPhase = null;
+        dwellDrift = 0;
         target = sign.targetId;
         face.userData.textureUpdateCount += 1;
         face.userData.fontReadyWhenDrawn =
@@ -516,7 +527,14 @@ export function createSign(
         initialized = false;
       }
     },
-    compose(camera, width, height, navigationBottom, now) {
+    compose(
+      camera,
+      width,
+      height,
+      navigationBottom,
+      now,
+      carAnchor = { x: 0, z: 0.72 },
+    ) {
       if (!snapshot) return;
       viewportWidth = width;
       viewportHeight = height;
@@ -546,6 +564,8 @@ export function createSign(
       const dt = Math.min(0.05, Math.max(0, (now - previousNow) / 1000));
       previousNow = now;
       const mix = initialized ? 1 - Math.exp(-dt / 0.18) : 1;
+      const roadside = usesRoadsideSign(width, height);
+      roadsideMix += ((roadside ? 1 : 0) - roadsideMix) * mix;
       screenX += (targetX - screenX) * mix;
       screenY += (targetY - screenY) * mix;
       screenWidth += (targetWidth - screenWidth) * mix;
@@ -583,9 +603,43 @@ export function createSign(
       worldPoint(worldLeft, cx - projectedWidth / 2, cy, depth, camera);
       worldPoint(worldRight, cx + projectedWidth / 2, cy, depth, camera);
       if (lowerStrip) point.y = Math.max(1.4, point.y);
+      // A wide-screen sign is a real roadside object: fixed lateral offset,
+      // fixed mast height, constant size, and travel only along the road axis.
+      // Camera movement changes its projection, never its world-space anchor.
+      const desiredPhysical = roadsideSignMetrics(height);
+      for (const key of ['x', 'y', 'scale', 'dwellZ'] as const) {
+        physical[key] += (desiredPhysical[key] - physical[key]) * mix;
+      }
+      // Canonical dwell progress is always 1; visual drift must integrate time.
+      if (snapshot.phase === 'dwell')
+        dwellDrift = Math.min(3, dwellDrift + dt * 1.7);
+      if (snapshot.phase === 'depart' && previousPhase !== 'depart')
+        departureDrift = dwellDrift;
+      previousPhase = snapshot.phase;
+      const roadZ =
+        snapshot.phase === 'depart'
+          ? THREE.MathUtils.lerp(physical.dwellZ + departureDrift, 48, p)
+          : THREE.MathUtils.lerp(-230, physical.dwellZ, arrival) + dwellDrift;
+      point.x = THREE.MathUtils.lerp(
+        point.x,
+        carAnchor.x + side * physical.x,
+        roadsideMix,
+      );
+      point.y = THREE.MathUtils.lerp(point.y, physical.y, roadsideMix);
+      point.z = THREE.MathUtils.lerp(point.z, carAnchor.z + roadZ, roadsideMix);
       group.position.set(point.x, point.y, point.z);
-      group.scale.setScalar(worldRight.distanceTo(worldLeft) / 8.64);
-      group.rotation.y = camera.rotation.y - side * 0.065;
+      group.scale.setScalar(
+        THREE.MathUtils.lerp(
+          worldRight.distanceTo(worldLeft) / 8.64,
+          physical.scale,
+          roadsideMix,
+        ),
+      );
+      group.rotation.y = THREE.MathUtils.lerp(
+        camera.rotation.y - side * 0.065,
+        -side * 0.1,
+        roadsideMix,
+      );
       plantMounting();
       const opacity = clamp01(snapshot.opacity);
       for (const [mat, weight] of fadeMaterials) mat.opacity = opacity * weight;
@@ -596,6 +650,7 @@ export function createSign(
       sweepMaterial.uniforms.uOpacity.value =
         snapshot.phase === 'approach' ? opacity * 0.14 : 0;
       group.visible = opacity > 0;
+      group.userData.anchoring = roadside ? 'roadside' : 'compact-shelf';
     },
   };
 }