feat(11-02): add step transition animation with reduced-motion guard

- Add @keyframes step-in (opacity 0->1, translateY 8px->0, 200ms ease-out) to index.css
- Register --animate-step-in token in @theme block
- Add @media prefers-reduced-motion: reduce guard disabling animation
- Add key={state.currentStep} and animate-step-in class to step wrapper in App.tsx

src/App.tsx

@@ -62,7 +62,7 @@ function WizardShell() {
         ) : (
           <>
             <StepIndicator />
-            <div className="mt-8">
+            <div key={state.currentStep} className="mt-8 animate-step-in">
               {CurrentStep}
             </div>
           </>

src/index.css

@@ -66,4 +66,22 @@
   --color-on-success:           var(--r2b-on-success);
   --color-warning:              var(--r2b-warning);
   --color-on-warning:           var(--r2b-on-warning);
+  --animate-step-in:            step-in 200ms ease-out both;
+}
+
+@keyframes step-in {
+  from {
+    opacity: 0;
+    transform: translateY(8px);
+  }
+  to {
+    opacity: 1;
+    transform: translateY(0);
+  }
+}
+
+@media (prefers-reduced-motion: reduce) {
+  .animate-step-in {
+    animation: none !important;
+  }
 }