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

uninstall.php

@@ -69,6 +69,10 @@ foreach ( $options as $option ) {
 $wpdb->query(
 	"DELETE FROM {$wpdb->postmeta} WHERE meta_key LIKE '\\_prautoblogger\\_%'"
 );
+// P2b.3 (v0.30.0): SEO stage writes _prab_* keys (different prefix — covered separately).
+$wpdb->query(
+	"DELETE FROM {$wpdb->postmeta} WHERE meta_key LIKE '\\_prab\\_%'"
+);
 
 /*
 |--------------------------------------------------------------------------