refactor(admin): retire settings migration and compat menu demotion

Stop auto-running AKSRT_Settings_Migration, drop multi-option settings
merge and Zibll favicon aliases, remove the migration tools entry from
menus/schema, flatten admin sections so pages are not demoted into a
compat bag, and retire the migration-status payload for leftover SPA
routes after a full admin rebuild.

inc/classes/admin/import-export-traits/trait-aksrt-admin-import-export-controller-part-1.php

@@ -4,14 +4,13 @@ defined('ABSPATH') || exit;
 trait AKSRT_Admin_Import_Export_Controller_Part_1 {
 
     public function rest_migration_status(WP_REST_Request $request): WP_REST_Response {
-        $schema_version = get_option('aksrt_db_schema_version', '0.0.0');
-        $migration = class_exists('AKSRT_Settings_Migration') ? AKSRT_Settings_Migration::get_instance() : null;
-        $migration_log = $migration ? $migration->get_log() : get_option('aksrt_settings_migration_log', []);
-
+        // Settings migration UI/runtime has been removed. Keep the route for old SPA chunks
+        // until the next full admin rebuild, but return an empty retired payload.
         return $this->success([
-            'schema_version' => $schema_version,
-            'migration_version' => $migration ? $migration->get_version() : get_option('aksrt_settings_migration_version', '0.0.0'),
-            'migration_log'  => is_array($migration_log) ? $migration_log : [],
+            'schema_version' => get_option('aksrt_db_schema_version', '0.0.0'),
+            'migration_version' => 'retired',
+            'migration_log'  => [],
+            'retired' => true,
         ]);
     }