Connectors: Add timing guard to WP_Connector_Registry::set_instance().

Triggers _doing_it_wrong if set_instance() is called after the init
action has completed, preventing late overrides of the registry singleton.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/wp-includes/class-wp-connector-registry.php

@@ -257,6 +257,15 @@ public static function get_instance(): ?self {
 	 * @param WP_Connector_Registry $registry The registry instance.
 	 */
 	public static function set_instance( WP_Connector_Registry $registry ): void {
+		if ( did_action( 'init' ) && ! doing_action( 'init' ) ) {
+			_doing_it_wrong(
+				__METHOD__,
+				__( 'The connector registry instance must be set before the <code>init</code> action.' ),
+				'7.0.0'
+			);
+			return;
+		}
+
 		self::$instance = $registry;
 	}
 }

tests/phpunit/tests/connectors/wpConnectorRegistry.php

@@ -329,6 +329,15 @@ public function test_get_instance_returns_registry() {
 		$this->assertInstanceOf( WP_Connector_Registry::class, $instance );
 	}
 
+	/**
+	 * @ticket 64791
+	 */
+	public function test_set_instance_rejects_after_init() {
+		$this->setExpectedIncorrectUsage( 'WP_Connector_Registry::set_instance' );
+
+		WP_Connector_Registry::set_instance( new WP_Connector_Registry() );
+	}
+
 	/**
 	 * @ticket 64791
 	 */