feat: add PR_Core_Related_Posts_Provider interface

includes/interfaces/interface-related-posts-provider.php

@@ -0,0 +1,21 @@
+<?php
+declare(strict_types=1);
+
+/**
+ * Interface for fetching related posts for a peptide.
+ *
+ * What: Defines the contract for retrieving posts related to a given peptide.
+ * Who calls it: PR_Core_Related_Posts_Section uses implementations to fetch related articles.
+ * Dependencies: None.
+ */
+interface PR_Core_Related_Posts_Provider {
+
+	/**
+	 * Get posts related to a peptide.
+	 *
+	 * @param int $peptide_id The peptide post ID.
+	 * @param int $limit      Maximum number of posts to return.
+	 * @return array<int, \WP_Post> Array of WP_Post objects (may be empty).
+	 */
+	public function get_posts( int $peptide_id, int $limit ): array;
+}