style: fade the exit plate in over 160 ms under reduced motion

src/styles/gantry.css

@@ -821,6 +821,14 @@ html[data-expressway='live'] .portfolio-section[data-layout='project-stage'] {
   html .journey-hud,
   html .site-navigation li::after { transition: none; }
 }
+/* No sign flies in under reduced motion, so the plate itself carries the
+   change of chapter: a 160 ms opacity fade, no movement. */
+@media (prefers-reduced-motion: reduce) {
+  html .journey-hud { animation: plate-swap 160ms ease-out both; }
+}
+@keyframes plate-swap {
+  from { opacity: 0; }
+}
 @media (forced-colors: active) {
   html .site-navigation::before { background: Canvas; }
   html .site-navigation::after { color: CanvasText; }