Add Polymorphic_Field_Helper for JSON-encoded polymorphic form fields

Fix incorrect data-sid selector in route-debug help example
Fix Form_Utils to use component.$sid() instead of data-sid selector
Add response helper functions and use _message as reserved metadata key

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

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

docs/CLAUDE.dist.md

@@ -924,6 +924,30 @@ public static function save(Request $request, array $params = []) {
 
 Details: `php artisan rsx:man form_conventions`
 
+### Polymorphic Form Fields
+
+For fields that can reference multiple model types (e.g., an Activity linked to either a Contact or Project), use JSON-encoded polymorphic values.
+
+```php
+use App\RSpade\Core\Polymorphic_Field_Helper;
+
+$eventable = Polymorphic_Field_Helper::parse($params['eventable'], [
+    Contact_Model::class,
+    Project_Model::class,
+]);
+
+if ($error = $eventable->validate('Please select an entity')) {
+    $errors['eventable'] = $error;
+}
+
+$model->eventable_type = $eventable->model;
+$model->eventable_id = $eventable->id;
+```
+
+Client submits: `{"model":"Contact_Model","id":123}`. Always use `Model::class` for the whitelist.
+
+Details: `php artisan rsx:man polymorphic`
+
 ---
 
 ## MODALS