feat(phase2b): P2b.4 — tier routing + Authority pipeline + citation/imagery gates (#181, v0.31.0)

Authority_Pipeline orchestrator (research→curate→draft→editorial→seo→publish gate) + per-category tier router behind master flag prautoblogger_authority_pipeline_enabled (DEFAULT OFF — zero behavior change until enabled). Cost-ceiling halts→hold; citation + imagery gates. QA APPROVE b086079; CI green (phpcs+phpunit VPS-verified).

Agent-Session: prab-phase2b-4-merge

includes/providers/interface-tier-router.php

@@ -0,0 +1,35 @@
+<?php
+/**
+ * Tier Router Interface
+ *
+ * Contract for resolving the generation tier (authority / economy) for an article idea.
+ *
+ * @package PRAutoBlogger
+ * @since 0.31.0
+ */
+
+declare(strict_types=1);
+
+if ( ! defined( 'ABSPATH' ) ) {
+	exit;
+}
+
+/**
+ * Resolves Authority vs Economy generation tier for a given article idea.
+ *
+ * Implementations read the per-category tier map and the master feature flag.
+ * When the master flag is OFF, resolve() MUST return 'economy' unconditionally
+ * to guarantee zero behaviour change in production.
+ *
+ * @see core/class-tier-router.php — Production implementation.
+ */
+interface PRAutoBlogger_Tier_Router_Interface {
+
+	/**
+	 * Resolve the generation tier for an article idea.
+	 *
+	 * @param PRAutoBlogger_Article_Idea $idea The article idea to route.
+	 * @return string 'authority' or 'economy'
+	 */
+	public function resolve( PRAutoBlogger_Article_Idea $idea ): string;
+}