feat(sync) : add slot selection controllers, custom field sync, and position fallbacks

- Add selectedPieceId support to ComposantPieceSlotController
- Create ComposantProductSlotController and ComposantSubcomponentSlotController
- Add updateCustomFields() to SkeletonStructureService for managing CustomField entities
- Fix position/orderIndex fallback to array index in all 3 sync strategies
- Fix type comparison in ProductSyncStrategy for dual format support
- Update CLAUDE.md with new entities, controllers, and fixtures documentation
- Update frontend submodule with interactive slot selectors

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/Controller/ComposantPieceSlotController.php

@@ -5,6 +5,7 @@ declare(strict_types=1);
 namespace App\Controller;
 
 use App\Entity\ComposantPieceSlot;
+use App\Entity\Piece;
 use Doctrine\ORM\EntityManagerInterface;
 use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
 use Symfony\Component\HttpFoundation\JsonResponse;
@@ -37,12 +38,22 @@ class ComposantPieceSlotController extends AbstractController
             $slot->setQuantity(max(1, (int) $payload['quantity']));
         }
 
+        if (array_key_exists('selectedPieceId', $payload)) {
+            if (null === $payload['selectedPieceId']) {
+                $slot->setSelectedPiece(null);
+            } else {
+                $piece = $this->entityManager->find(Piece::class, $payload['selectedPieceId']);
+                $slot->setSelectedPiece($piece);
+            }
+        }
+
         $this->entityManager->flush();
 
         return $this->json([
-            'success'  => true,
-            'id'       => $slot->getId(),
-            'quantity' => $slot->getQuantity(),
+            'success'         => true,
+            'id'              => $slot->getId(),
+            'quantity'        => $slot->getQuantity(),
+            'selectedPieceId' => $slot->getSelectedPiece()?->getId(),
         ]);
     }
 }