fix(data-integrity) : prevent data loss in clone, slots, conversion and custom fields

- Clone: CustomFieldValue now references cloned CustomField, not source
- Slots: validate piece type matches slot requirement + 404 on missing piece
- Conversion: check slot tables before allowing category conversion + clean orphan skeleton requirements
- CustomFieldValue: prevent creation of orphan CustomField without target entity

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

src/Controller/MachineStructureController.php

@@ -173,6 +173,8 @@ class MachineStructureController extends AbstractController
 
     private function cloneCustomFields(Machine $source, Machine $target): void
     {
+        $cfMap = [];
+
         foreach ($source->getCustomFields() as $cf) {
             $newCf = new CustomField();
             $newCf->setName($cf->getName());
@@ -183,12 +185,20 @@ class MachineStructureController extends AbstractController
             $newCf->setOrderIndex($cf->getOrderIndex());
             $newCf->setMachine($target);
             $this->entityManager->persist($newCf);
+
+            $cfMap[$cf->getId()] = $newCf;
         }
 
         foreach ($source->getCustomFieldValues() as $cfv) {
+            $originalCf = $cfv->getCustomField();
+            $newCf      = $cfMap[$originalCf->getId()] ?? null;
+            if (!$newCf) {
+                continue;
+            }
+
             $newValue = new CustomFieldValue();
             $newValue->setMachine($target);
-            $newValue->setCustomField($cfv->getCustomField());
+            $newValue->setCustomField($newCf);
             $newValue->setValue($cfv->getValue());
             $this->entityManager->persist($newValue);
         }