feat(widgets): add phase 4 validation and accessibility

src/WidgetSystem/WidgetVisibility.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace AKSRT\WidgetSystem;
+
+defined('ABSPATH') || exit;
+
+/** Pure visibility rules shared by rendering and contract tests. */
+final class WidgetVisibility {
+	public static function should_render(array $instance, ?int $current_id = null, ?bool $singular = null): bool {
+		$type = (string) ($instance['show_id_type'] ?? '');
+		$ids = RendererHelpers::ids((string) ($instance['show_ids'] ?? ''));
+		if ($type === '' || !$ids) {
+			return true;
+		}
+
+		if ($singular === null) {
+			$singular = function_exists('is_singular') && is_singular();
+		}
+		if ($current_id === null) {
+			$current_id = $singular && function_exists('get_queried_object_id') ? (int) get_queried_object_id() : 0;
+		}
+		$matched = $singular && in_array($current_id, $ids, true);
+		return $type === 'show' ? $matched : !$matched;
+	}
+
+	public static function device_class(array $instance): string {
+		$type = (string) ($instance['show_type'] ?? 'all');
+		if ($type === 'only_pc') {
+			return 'aksrt-widget--only-pc';
+		}
+		return $type === 'only_sm' ? 'aksrt-widget--only-mobile' : '';
+	}
+}