Feature: Add ManifestResult DTO for carrier manifest batching

Completes the carrier-agnostic shipping abstraction (CarrierFulfillmentInterface, SupportsManifestBatching, Shipment model) with the result type SupportsManifestBatching::issueManifest() returns.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/Shipping/DataTransferObjects/ManifestResult.php

@@ -0,0 +1,26 @@
+<?php
+
+namespace Modules\Core\Shipping\DataTransferObjects;
+
+use Illuminate\Support\Collection;
+
+class ManifestResult
+{
+    private function __construct(
+        public readonly bool $success,
+        public readonly ?string $reference,
+        public readonly Collection $includedShipments,
+        public readonly Collection $blockedShipments,
+        public readonly ?string $reason,
+    ) {}
+
+    public static function success(string $reference, Collection $includedShipments): self
+    {
+        return new self(true, $reference, $includedShipments, collect(), null);
+    }
+
+    public static function blocked(Collection $blockedShipments, string $reason): self
+    {
+        return new self(false, null, collect(), $blockedShipments, $reason);
+    }
+}