Add root() method to extension traits

src/Concerns/HasDomainExtensions.php

···
     abstract protected function getRootClientClass(): string;
 
     /**
+     * Get the root client instance.
+     */
+    abstract public function root(): object;
+
+    /**
      * Resolve a domain extension instance.
      */
     protected function resolveDomainExtension(string $name): object

src/Concerns/HasExtensions.php

···
     {
         return static::hasExtension($name);
     }
+
+    /**
+     * Get the root client instance.
+     *
+     * For root clients, this returns itself.
+     * Domain clients override this to return their parent.
+     */
+    public function root(): static
+    {
+        return $this;
+    }
 }