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/ComposantProductSlotController.php

@@ -0,0 +1,54 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Controller;
+
+use App\Entity\ComposantProductSlot;
+use App\Entity\Product;
+use Doctrine\ORM\EntityManagerInterface;
+use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
+use Symfony\Component\HttpFoundation\JsonResponse;
+use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\Routing\Attribute\Route;
+
+#[Route('/api/composant-product-slots')]
+class ComposantProductSlotController extends AbstractController
+{
+    public function __construct(
+        private readonly EntityManagerInterface $entityManager,
+    ) {}
+
+    #[Route('/{id}', name: 'composant_product_slot_patch', methods: ['PATCH'])]
+    public function patch(string $id, Request $request): JsonResponse
+    {
+        $this->denyAccessUnlessGranted('ROLE_GESTIONNAIRE');
+
+        $slot = $this->entityManager->find(ComposantProductSlot::class, $id);
+        if (!$slot) {
+            return $this->json(['success' => false, 'error' => 'Slot not found.'], 404);
+        }
+
+        $payload = json_decode($request->getContent(), true);
+        if (!is_array($payload)) {
+            return $this->json(['success' => false, 'error' => 'Invalid JSON payload.'], 400);
+        }
+
+        if (array_key_exists('selectedProductId', $payload)) {
+            if (null === $payload['selectedProductId']) {
+                $slot->setSelectedProduct(null);
+            } else {
+                $product = $this->entityManager->find(Product::class, $payload['selectedProductId']);
+                $slot->setSelectedProduct($product);
+            }
+        }
+
+        $this->entityManager->flush();
+
+        return $this->json([
+            'success'           => true,
+            'id'                => $slot->getId(),
+            'selectedProductId' => $slot->getSelectedProduct()?->getId(),
+        ]);
+    }
+}