Make generator template precompile hook use shared implementation when available

Update the generator template to intelligently use the shared precompile hook
implementation when running in the gem's development/test context, while
maintaining a self-contained fallback for generated user applications.

This provides the best of both worlds:
- Test dummy apps and generator template all use the same shared code
- Generated user apps have a standalone copy that works independently
- Easier to maintain and update the precompile hook logic

The hook now tries to load the shared implementation from the gem first,
and falls back to its inline implementation if the gem isn't available
or accessible.

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

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

Author: justin808

lib/generators/react_on_rails/templates/base/base/bin/shakapacker-precompile-hook

@@ -11,6 +11,22 @@
 #
 # See: https://github.com/shakacode/shakapacker/blob/main/docs/precompile_hook.md
 
+# Try to use shared implementation from gem if available (during development/testing)
+# This allows the gem's test apps and generated apps to share the same implementation
+begin
+  require "react_on_rails"
+  gem_root = Gem.loaded_specs["react_on_rails"]&.gem_dir
+  shared_hook = File.join(gem_root, "lib", "tasks", "precompile", "shakapacker_precompile_hook_shared.rb") if gem_root
+
+  if shared_hook && File.exist?(shared_hook)
+    load shared_hook
+    exit 0
+  end
+rescue LoadError, StandardError
+  # If we can't load from gem, fall through to inline implementation
+end
+
+# Inline implementation for generated apps (when gem source isn't available)
 require "fileutils"
 
 # Find Rails root by walking upward looking for config/environment.rb