fix(composant) : persist piece/product/subcomponent selections on creation

The ComposantProcessor now reads the structure payload from the frontend
and applies selectedPieceId/selectedProductId/selectedComponentId to the
scaffolded slots, so user selections are actually saved to the database.

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

src/Entity/Composant.php

@@ -131,6 +131,12 @@ class Composant
     #[ORM\OrderBy(['position' => 'ASC'])]
     private Collection $productSlots;
 
+    /**
+     * Transient — holds the structure payload sent by the frontend during creation.
+     * Not mapped to any column; consumed by ComposantProcessor.
+     */
+    private ?array $pendingStructure = null;
+
     #[ORM\Column(type: Types::INTEGER, options: ['default' => 1])]
     #[Groups(['composant:read'])]
     private int $version = 1;
@@ -395,6 +401,27 @@ class Composant
         ];
     }
 
+    /**
+     * Called by API Platform during denormalization — stores the frontend
+     * structure payload so the ComposantProcessor can apply selections.
+     */
+    public function setStructure(?array $structure): static
+    {
+        $this->pendingStructure = $structure;
+
+        return $this;
+    }
+
+    public function getPendingStructure(): ?array
+    {
+        return $this->pendingStructure;
+    }
+
+    public function clearPendingStructure(): void
+    {
+        $this->pendingStructure = null;
+    }
+
     public function addProductSlot(ComposantProductSlot $productSlot): static
     {
         if (!$this->productSlots->contains($productSlot)) {