docs(Suspense): warn against module-level cache in SSR

Add a Pitfall on the Suspense reference page explaining that the
hidden data.js cache must not be copied into server environments like
Next.js, where module-level state can leak data across requests.

Clarify the data.js comment in all Suspense examples accordingly.

Fixes #8134

Author: cursoragent

src/content/reference/react/Suspense.md

@@ -79,6 +79,12 @@ React will display your <CodeStep step={1}>loading fallback</CodeStep> until all
 
 In the example below, the `Albums` component *suspends* while fetching the list of albums. Until it's ready to render, React switches the closest Suspense boundary above to show the fallback--your `Loading` component. Then, when the data loads, React hides the `Loading` fallback and renders the `Albums` component with data.
 
+<Pitfall>
+
+Do not use a module-level cache like the one in `data.js` below in server environments such as Next.js. Module-level state persists across requests and can leak data between users. Use your framework's built-in caching, [`cache`](/reference/react/cache) in Server Components, or scope caches per request (for example, with a Context and `useRef`).
+
+</Pitfall>
+
 <Sandpack>
 
 ```js src/App.js hidden
@@ -148,6 +154,8 @@ export default function Albums({ artistId }) {
 // Note: the way you would do data fetching depends on
 // the framework that you use together with Suspense.
 // Normally, the caching logic would be inside a framework.
+// This module-level cache is only suitable for this client-only demo.
+// Do not copy it into server environments.
 
 let cache = new Map();
 
@@ -606,6 +614,8 @@ export default function Albums({ artistId }) {
 // Note: the way you would do data fetching depends on
 // the framework that you use together with Suspense.
 // Normally, the caching logic would be inside a framework.
+// This module-level cache is only suitable for this client-only demo.
+// Do not copy it into server environments.
 
 let cache = new Map();
 
@@ -868,6 +878,8 @@ export default function Albums({ artistId }) {
 // Note: the way you would do data fetching depends on
 // the framework that you use together with Suspense.
 // Normally, the caching logic would be inside a framework.
+// This module-level cache is only suitable for this client-only demo.
+// Do not copy it into server environments.
 
 let cache = new Map();
 
@@ -1052,6 +1064,8 @@ export default function SearchResults({ query }) {
 // Note: the way you would do data fetching depends on
 // the framework that you use together with Suspense.
 // Normally, the caching logic would be inside a framework.
+// This module-level cache is only suitable for this client-only demo.
+// Do not copy it into server environments.
 
 let cache = new Map();
 
@@ -1236,6 +1250,8 @@ export default function SearchResults({ query }) {
 // Note: the way you would do data fetching depends on
 // the framework that you use together with Suspense.
 // Normally, the caching logic would be inside a framework.
+// This module-level cache is only suitable for this client-only demo.
+// Do not copy it into server environments.
 
 let cache = new Map();
 
@@ -1495,6 +1511,8 @@ export default function Panel({ children }) {
 // Note: the way you would do data fetching depends on
 // the framework that you use together with Suspense.
 // Normally, the caching logic would be inside a framework.
+// This module-level cache is only suitable for this client-only demo.
+// Do not copy it into server environments.
 
 let cache = new Map();
 
@@ -1807,6 +1825,8 @@ export default function Panel({ children }) {
 // Note: the way you would do data fetching depends on
 // the framework that you use together with Suspense.
 // Normally, the caching logic would be inside a framework.
+// This module-level cache is only suitable for this client-only demo.
+// Do not copy it into server environments.
 
 let cache = new Map();
 
@@ -2118,6 +2138,8 @@ export default function Panel({ children }) {
 // Note: the way you would do data fetching depends on
 // the framework that you use together with Suspense.
 // Normally, the caching logic would be inside a framework.
+// This module-level cache is only suitable for this client-only demo.
+// Do not copy it into server environments.
 
 let cache = new Map();