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/Service/Sync/ProductSyncStrategy.php

@@ -43,10 +43,11 @@ class ProductSyncStrategy implements SyncStrategyInterface
             $existingByOrder[$field->getOrderIndex()] = $field;
         }
 
-        // Map proposed fields by orderIndex
+        // Map proposed fields by orderIndex (falls back to array index)
         $proposedByOrder = [];
-        foreach ($proposedFields as $pf) {
-            $proposedByOrder[$pf['orderIndex']] = $pf;
+        foreach ($proposedFields as $i => $pf) {
+            $order                   = $pf['orderIndex'] ?? $i;
+            $proposedByOrder[$order] = $pf;
         }
 
         $addedFields    = 0;
@@ -57,7 +58,7 @@ class ProductSyncStrategy implements SyncStrategyInterface
         foreach ($proposedByOrder as $orderIndex => $pf) {
             if (!isset($existingByOrder[$orderIndex])) {
                 ++$addedFields;
-            } elseif ($existingByOrder[$orderIndex]->getType() !== $pf['type']) {
+            } elseif ($existingByOrder[$orderIndex]->getType() !== ($pf['type'] ?? $pf['value']['type'] ?? null)) {
                 ++$modifiedFields;
             }
         }