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/ComposantPieceSlotController.php

@@ -43,6 +43,18 @@ class ComposantPieceSlotController extends AbstractController
                 $slot->setSelectedPiece(null);
             } else {
                 $piece = $this->entityManager->find(Piece::class, $payload['selectedPieceId']);
+                if (!$piece) {
+                    return $this->json(['success' => false, 'error' => 'Pièce introuvable.'], 404);
+                }
+
+                $slotTypePiece = $slot->getTypePiece();
+                if ($slotTypePiece && $piece->getTypePiece()?->getId() !== $slotTypePiece->getId()) {
+                    return $this->json([
+                        'success' => false,
+                        'error'   => sprintf('La pièce doit être de type « %s ».', $slotTypePiece->getName()),
+                    ], 422);
+                }
+
                 $slot->setSelectedPiece($piece);
             }
         }