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>

config/reference.php

@@ -1626,6 +1626,19 @@ use Symfony\Component\Config\Loader\ParamConfigurator as Param;
  *     nelmio_cors?: NelmioCorsConfig,
  *     api_platform?: ApiPlatformConfig,
  *     lexik_jwt_authentication?: LexikJwtAuthenticationConfig,
+ *     "when@dev"?: array{
+ *         imports?: ImportsConfig,
+ *         parameters?: ParametersConfig,
+ *         services?: ServicesConfig,
+ *         framework?: FrameworkConfig,
+ *         twig?: TwigConfig,
+ *         security?: SecurityConfig,
+ *         doctrine?: DoctrineConfig,
+ *         doctrine_migrations?: DoctrineMigrationsConfig,
+ *         nelmio_cors?: NelmioCorsConfig,
+ *         api_platform?: ApiPlatformConfig,
+ *         lexik_jwt_authentication?: LexikJwtAuthenticationConfig,
+ *     },
  *     "when@prod"?: array{
  *         imports?: ImportsConfig,
  *         parameters?: ParametersConfig,
@@ -1732,6 +1745,7 @@ namespace Symfony\Component\Routing\Loader\Configurator;
  *     deprecated?: array{package:string, version:string, message?:string},
  * }
  * @psalm-type RoutesConfig = array{
+ *     "when@dev"?: array<string, RouteConfig|ImportConfig|AliasConfig>,
  *     "when@prod"?: array<string, RouteConfig|ImportConfig|AliasConfig>,
  *     "when@test"?: array<string, RouteConfig|ImportConfig|AliasConfig>,
  *     ...<string, RouteConfig|ImportConfig|AliasConfig>