Add environment variable control for testing loading strategies

Makes it easy to reproduce CI failures locally by testing with different
script loading strategies (async vs defer) without code changes.

Changes:
- spec/dummy/config/initializers/react_on_rails.rb: Use REACT_ON_RAILS_LOADING_STRATEGY env var
- spec/dummy/TESTING_LOCALLY.md: Add section on reproducing race condition failures

Usage:
  # Test with defer (default, prevents race conditions)
  bundle exec rspec spec/system/integration_spec.rb -e "shared_store"

  # Test with async (library default for users, may show race conditions)
  env REACT_ON_RAILS_LOADING_STRATEGY=async bundle exec rspec spec/system/integration_spec.rb -e "shared_store"

This eliminates the need for .example files and provides a simple,
documented way to test both loading strategies locally.

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

Co-Authored-By: Claude <[email protected]>

Author: justin808

spec/dummy/TESTING_LOCALLY.md

@@ -1,5 +1,50 @@
 # Testing Locally
 
+## Reproducing CI Failures with Component Registration Race Conditions
+
+If you see CI failures like "Could not find component registered with name ReduxSharedStoreApp", this is due to a race condition with async script loading. Here's how to reproduce and fix it locally:
+
+### Quick Reproduction
+
+```bash
+cd spec/dummy
+
+# Test with async loading (CI default for library users)
+REACT_ON_RAILS_LOADING_STRATEGY=async bundle exec rspec spec/system/integration_spec.rb -e "shared_store"
+
+# If that passes but CI fails, run multiple times to catch intermittent failures:
+for i in {1..5}; do
+  echo "Run $i:"
+  REACT_ON_RAILS_LOADING_STRATEGY=async bundle exec rspec spec/system/integration_spec.rb:376 spec/system/integration_spec.rb:380
+done
+```
+
+### Understanding the Issue
+
+- **The dummy app defaults to `:defer`** to prevent race conditions during development
+- **Real user apps default to `:async`** (when Shakapacker >= 8.2.0) for better performance
+- **The race condition**: With async, `client-bundle.js` can execute before generated component packs, causing components to be missing from the registry when React tries to hydrate
+
+### The Fix
+
+The dummy app uses an environment variable to control the loading strategy:
+
+```ruby
+# spec/dummy/config/initializers/react_on_rails.rb
+loading_strategy = ENV.fetch("REACT_ON_RAILS_LOADING_STRATEGY", "defer").to_sym
+config.generated_component_packs_loading_strategy = loading_strategy
+```
+
+This allows testing both strategies without code changes.
+
+### When CI Fails But Local Passes
+
+1. **Check the loading strategy**: CI may be testing with `:async` while you're testing with `:defer`
+2. **Run with async locally**: Use the environment variable to match CI conditions
+3. **If it's intermittent**: The race condition timing varies - run tests multiple times
+
+### SSL Issues (Not Related to Component Registration)
+
 ## Known Issues with Ruby 3.4.3 + OpenSSL 3.6
 
 If you're running Ruby 3.4.3 with OpenSSL 3.6+, you may encounter SSL certificate verification errors in system tests:

spec/dummy/config/initializers/react_on_rails.rb

@@ -42,5 +42,10 @@ def self.adjust_props_for_client_side_hydration(component_name, props)
   config.components_subdirectory = "startup"
   config.auto_load_bundle = true
   config.immediate_hydration = false
-  config.generated_component_packs_loading_strategy = :defer
+
+  # Use :defer to prevent race conditions with auto-generated component packs.
+  # To test with :async (CI default), set: REACT_ON_RAILS_LOADING_STRATEGY=async
+  # See: spec/dummy/TESTING_LOCALLY.md for reproducing CI failures
+  loading_strategy = ENV.fetch("REACT_ON_RAILS_LOADING_STRATEGY", "defer").to_sym
+  config.generated_component_packs_loading_strategy = loading_strategy
 end