Fix CI: Revert OSS dummy to defer: true for Redux compatibility

The OSS dummy app uses Redux shared stores with inline component
registration, which requires defer: true. With async: true, the bundle
executes before inline scripts, causing component registration failures.

Changes:
- spec/dummy: Reverted to defer: true (was async: true)
- Updated comment to explain Redux inline registration requirement
- Pro dummy: Remains async: true (has getOrWaitForStore support)

This fixes CI test failures:
- "Could not find component registered with name ReduxApp"
- "Could not find store registered with name 'SharedReduxStore'"

Root cause: async scripts execute immediately upon download, before
inline <script> tags in HTML. defer scripts wait for HTML parsing,
ensuring proper execution order.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: justin808

spec/dummy/app/views/layouts/application.html.erb

@@ -9,10 +9,11 @@
 
   <%= yield :head %>
 
-  <%# async: true is the recommended approach per docs/building-features/streaming-server-rendering.md.
-      It enables React 18's Selective Hydration and provides optimal Time to Interactive (TTI).
-      Requires Shakapacker >= 8.2.0 (currently using 9.3.0). %>
-  <%= javascript_pack_tag('client-bundle', 'data-turbo-track': 'reload', async: true) %>
+  <%# defer: true is required because this app uses Redux shared stores with inline component
+      registration. With async: true, the bundle executes before inline <script> tags, causing
+      "Could not find component" errors. See docs/building-features/streaming-server-rendering.md
+      for details. Modern apps should move registration to bundles and use async: true. %>
+  <%= javascript_pack_tag('client-bundle', 'data-turbo-track': 'reload', defer: true) %>
 
   <%= csrf_meta_tags %>
 </head>