feat(phase2b): P2b.3 — SEO stage (_prab_ meta + citation_score) (#180, v0.30.0)

Additive: seo stage writes _prab_* meta per the ratified prcore JSON-LD contract (emitter consumes); computes citation_score + configurable threshold (gate is P2b.4). Deterministic, no LLM/cost, live path untouched. Uninstall purges _prab_ meta. QA APPROVE 55b94d0; CI green (VPS-verified).

Agent-Session: prab-phase2b-3-merge

includes/providers/interface-seo-stage.php

@@ -0,0 +1,35 @@
+<?php
+declare(strict_types=1);
+
+/**
+ * Contract for the SEO stage of the Authority pipeline.
+ *
+ * Implementations write _prab_* post-meta per the ratified JSON-LD contract
+ * (convo/prcore/decisions/2026-06-11-jsonld-contract-v1.md) and compute
+ * citation_score from kept research sources.
+ *
+ * @see core/class-seo-stage.php — Production implementation.
+ */
+interface PRAutoBlogger_Seo_Stage_Interface {
+
+	/**
+	 * Execute the SEO stage for a published article.
+	 *
+	 * Writes _prab_* post-meta, computes + stores citation_score, records
+	 * run_stages start→done and a run_decisions row.
+	 *
+	 * @param string $run_id      Pipeline run UUID.
+	 * @param string $item_key    Article-scoped stage key.
+	 * @param int    $post_id     Published WordPress post ID.
+	 * @param array<int, array{url: string, title: string, doi?: string, quality_score?: float}> $kept_sources Kept research sources from the curate stage.
+	 * @param array<int, int> $peptide_ids Related peptide post IDs (may be empty).
+	 * @return float The computed citation_score (0.0–1.0).
+	 */
+	public function run(
+		string $run_id,
+		string $item_key,
+		int $post_id,
+		array $kept_sources,
+		array $peptide_ids
+	): float;
+}