enforce element level hierarchy

backend/app/Element/Element.php

@@ -3,14 +3,14 @@
 namespace App\Element;
 
 use App\Set\Set;
+use App\Set\SetLevel;
 
 final readonly class Element
 {
     public function __construct(
         private int $id,
         private string $name,
-        private string $kind,
-        private Set $set,
+        private SetLevel $level,
         private ?Element $parentElement,
         private int $position,
     ) {}
@@ -27,12 +27,17 @@ final readonly class Element
 
     public function getKind(): string
     {
-        return $this->kind;
+        return $this->level->getKind();
+    }
+
+    public function getLevel(): SetLevel
+    {
+        return $this->level;
     }
 
     public function getSet(): Set
     {
-        return $this->set;
+        return $this->level->getSet();
     }
 
     public function getParentElement(): ?Element