Update defer comment to clarify backward-compatibility only

Per reviewer guidance, updated comment in spec/dummy layout to explicitly
state that defer: true is retained solely for backward-compatibility and
testing purposes, not recommended for production.

The comment now references docs/building-features/streaming-server-rendering.md
and recommends using async: true with immediate_hydration or
generated_component_packs_loading_strategy: :async for production apps.

🤖 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 %>
 
-  <%# defer: true is safe here because this app does NOT use streaming server rendering.
-      defer also ensures scripts load in the correct order when using auto-generated component packs.
-      For apps with streaming components, use defer: false or async: true to enable
-      React 18's Selective Hydration. See docs/building-features/streaming-server-rendering.md %>
+  <%# defer: true is used here solely for backward-compatibility and testing purposes.
+      Per reviewer guidance and docs/building-features/streaming-server-rendering.md,
+      defer: true is NOT recommended for production in any scenario (streaming or non-streaming).
+      Instead, use async: true with immediate_hydration or set
+      generated_component_packs_loading_strategy: :async for production apps. %>
   <%= javascript_pack_tag('client-bundle', 'data-turbo-track': 'reload', defer: true) %>
 
   <%= csrf_meta_tags %>