Extract React version check to separate script file

Address code review feedback:
- Move inline version detection to scripts/check-react-version.cjs for
  better clarity and maintainability
- Use .cjs extension since the parent package is ESM and we need require()
- Add root Gemfile.lock to .gitignore (it's a delegate to react_on_rails/Gemfile)

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

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

Author: justin808

.gitignore

@@ -1,3 +1,6 @@
+# Root Gemfile.lock (delegates to react_on_rails/Gemfile, lock is there)
+/Gemfile.lock
+
 .bundle/
 /.yardoc
 /_yardoc/

packages/react-on-rails-pro/scripts/check-react-version.cjs

@@ -0,0 +1,19 @@
+/**
+ * Check if React version supports RSC features (React 19+).
+ * Exits with code 0 if React < 19 (skip RSC tests).
+ * Exits with code 1 if React >= 19 (run RSC tests).
+ *
+ * This is a CommonJS file (.cjs) because it needs to use require()
+ * and the parent package has "type": "module".
+ */
+const v = require('react/package.json').version;
+
+const majorVersion = parseInt(v, 10);
+
+if (majorVersion < 19) {
+  console.log(`RSC tests skipped (requires React 19+, found ${v})`);
+  process.exit(0);
+}
+
+// Exit with code 1 so the || chain continues to run Jest
+process.exit(1);