feat: sign interlude, window plate and a two-button control bay

The sign closes in over the road on a gantry truss, arcs into a wider
window and hands off to a flat 2D plate that mirrors its face; content
steps aside for the interlude and rises after. The bay keeps sound and
settings; volume and Pause motion move into the settings popover.

src/journey/sign-routes.ts

@@ -16,6 +16,18 @@ export interface SignRouteSpec {
   dwellOffsetX?: number;
 }
 
+/** Each exit's sign colour, shared by the 3D sign and the 2D plates. */
+export const SIGN_ACCENTS: Readonly<Record<SectionId, string>> = {
+  start: '#ff83c2',
+  profile: '#ff83c2',
+  experience: '#c4a2ff',
+  infrastructure: '#8be9ed',
+  'selected-builds': '#ffa68d',
+  'additional-work': '#ff83c2',
+  toolkit: '#c4a2ff',
+  contact: '#ffa68d',
+};
+
 export const SIGN_LIFECYCLE = Object.freeze({
   anticipateMs: 300,
   approachMs: 900,