feat: add peptide_topic taxonomy registration

includes/cpt/class-pr-core-peptide-cpt.php

@@ -2,9 +2,11 @@
 declare(strict_types=1);
 
 /**
- * Registers the peptide custom post type and associated taxonomy.
+ * Registers the peptide custom post type and associated taxonomies.
  *
  * What: Defines the peptide CPT with REST support, archive, and monograph meta fields.
+ *       Also registers the peptide_category taxonomy (for peptide categorization) and
+ *       peptide_topic taxonomy (for linking blog posts to peptides).
  * Who calls it: PR_Core::init() on plugins_loaded.
  * Dependencies: None.
  *
@@ -15,6 +17,10 @@ declare(strict_types=1);
  * deploy order between the two plugins does not matter — whichever runs first
  * wins, the other no-ops.
  *
+ * As of v0.3.0, `peptide_topic` taxonomy is registered for linking blog posts
+ * to peptides via a shared topic slug (e.g., 'bpc-157'). This enables the
+ * Related Articles feature.
+ *
  * @see ARCHITECTURE.md — CPT specification and post-meta field definitions.
  * @see CONVENTIONS.md — CPT ownership rule (no plugin registers a CPT another plugin owns).
  */
@@ -26,6 +32,9 @@ class PR_Core_Peptide_CPT {
 	/** @var string Taxonomy: category (e.g., GLP-1 agonist). */
 	public const TAX_CATEGORY = 'peptide_category';
 
+	/** @var string Taxonomy: topic (links blog posts to peptides by slug). */
+	public const TAX_TOPIC = 'peptide_topic';
+
 	/** @var string Capability required for editing peptide data. */
 	public const CAPABILITY = 'manage_peptide_content';
 
@@ -198,38 +207,58 @@ class PR_Core_Peptide_CPT {
 	}
 
 	/**
-	 * Register the `peptide_category` taxonomy.
+	 * Register the `peptide_category` and `peptide_topic` taxonomies.
 	 *
 	 * Guarded with `taxonomy_exists()` for the same reason CPT registration
-	 * is guarded. The 8 existing terms + term_relationships stay intact —
+	 * is guarded. The 8 existing category terms + term_relationships stay intact —
 	 * they key on taxonomy name `peptide_category` in wp_term_taxonomy,
 	 * which is exactly what we register here.
 	 *
 	 * v0.2.0: `pr_peptide_family` taxonomy removed — never populated, never
 	 * surfaced in UI.
 	 *
-	 * Side effects: registers taxonomy with WordPress.
+	 * v0.3.0: `peptide_topic` taxonomy added. Non-hierarchical, linked to posts,
+	 * uses peptide slugs as terms (e.g., 'bpc-157', 'tb-500'). Enables the Related
+	 * Articles feature for blog posts tagged with a peptide topic.
+	 *
+	 * Side effects: registers taxonomies with WordPress.
 	 *
 	 * @return void
 	 */
 	public static function register_taxonomies(): void {
-		if ( taxonomy_exists( self::TAX_CATEGORY ) ) {
-			return;
+		// Register peptide_category taxonomy.
+		if ( ! taxonomy_exists( self::TAX_CATEGORY ) ) {
+			register_taxonomy( self::TAX_CATEGORY, self::POST_TYPE, [
+				'labels'             => [
+					'name'          => __( 'Peptide Categories', 'peptide-repo-core' ),
+					'singular_name' => __( 'Peptide Category', 'peptide-repo-core' ),
+				],
+				'public'             => true,
+				'publicly_queryable' => true,
+				'show_in_rest'       => true,
+				'show_ui'            => true,
+				'show_admin_column'  => true,
+				'hierarchical'       => true,
+				'rewrite'            => [ 'slug' => 'peptide-category', 'with_front' => false ],
+			] );
 		}
 
-		register_taxonomy( self::TAX_CATEGORY, self::POST_TYPE, [
-			'labels'             => [
-				'name'          => __( 'Peptide Categories', 'peptide-repo-core' ),
-				'singular_name' => __( 'Peptide Category', 'peptide-repo-core' ),
-			],
-			'public'             => true,
-			'publicly_queryable' => true,
-			'show_in_rest'       => true,
-			'show_ui'            => true,
-			'show_admin_column'  => true,
-			'hierarchical'       => true,
-			'rewrite'            => [ 'slug' => 'peptide-category', 'with_front' => false ],
-		] );
+		// Register peptide_topic taxonomy (for blog post tagging).
+		if ( ! taxonomy_exists( self::TAX_TOPIC ) ) {
+			register_taxonomy( self::TAX_TOPIC, 'post', [
+				'labels'             => [
+					'name'          => __( 'Peptide Topics', 'peptide-repo-core' ),
+					'singular_name' => __( 'Peptide Topic', 'peptide-repo-core' ),
+				],
+				'public'             => true,
+				'publicly_queryable' => true,
+				'show_in_rest'       => true,
+				'show_ui'            => true,
+				'show_admin_column'  => true,
+				'hierarchical'       => false,
+				'rewrite'            => [ 'slug' => 'peptide-topic', 'with_front' => false ],
+			] );
+		}
 	}
 
 	/**