fix(custom-fields) : persist machineContextOnly in structure save and clone

- SkeletonStructureService: read and write machineContextOnly on create/update
- normalizeCustomFieldData: pass machineContextOnly through both payload formats
- cloneCustomFields: copy machineContextOnly flag on machine clone

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/Controller/MachineStructureController.php

@@ -191,6 +191,7 @@ class MachineStructureController extends AbstractController
             $newCf->setDefaultValue($cf->getDefaultValue());
             $newCf->setOptions($cf->getOptions());
             $newCf->setOrderIndex($cf->getOrderIndex());
+            $newCf->setMachineContextOnly($cf->isMachineContextOnly());
             $newCf->setMachine($target);
             $this->entityManager->persist($newCf);