Polish configuration and improve code clarity

## Changes

- **`.rubocop.yml`**: Exclude `bin/*` shell scripts from RuboCop checks
- **`lib/react_on_rails/configuration.rb`**: Fix error message wording (prefer :defer over :sync, remove extra comma and "performance issues" phrase)
- **`spec/dummy/spec/helpers/react_on_rails_helper_spec.rb`**: Add clarifying comment about defer default strategy

Small polish changes that improve code clarity and fix minor inconsistencies.

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

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

Author: justin808

.rubocop.yml

@@ -25,6 +25,7 @@ AllCops:
     - '**/node_modules/**/*'
     - '**/public/**/*'
     - '**/tmp/**/*'
+    - 'bin/*'  # Shell scripts
     - 'coverage/**/*'
     - 'gen-examples/examples/**/*'
     - 'node_modules/**/*'

lib/react_on_rails/configuration.rb

@@ -172,8 +172,8 @@ def validate_generated_component_packs_loading_strategy
 
       msg = <<~MSG
         ReactOnRails: Your current version of shakapacker \
-        does not support async script loading,  which may cause performance issues. Please either:
-        1. Use :sync or :defer loading strategy instead of :async
+        does not support async script loading. Please either:
+        1. Use :defer or :sync loading strategy instead of :async
         2. Upgrade to Shakapacker v8.2.0 or above to enable async script loading
       MSG
       if generated_component_packs_loading_strategy.nil?

spec/dummy/spec/helpers/react_on_rails_helper_spec.rb

@@ -77,6 +77,8 @@ def self.pro_attribution_comment
       allow(helper).to receive(:append_stylesheet_pack_tag)
       expect { helper.load_pack_for_generated_component("component_name", render_options) }.not_to raise_error
 
+      # Default loading strategy is now always :defer to prevent race conditions
+      # between component registration and hydration, regardless of async support
       expect(helper).to have_received(:append_javascript_pack_tag).with("generated/component_name", { defer: true })
       expect(helper).to have_received(:append_stylesheet_pack_tag).with("generated/component_name")
     end