feat(wpmc S2): make Plugin::boot() idempotent

Add private \$booted flag; guard skips all re-registration on subsequent
calls. TDD: RED (3x boot -> expectAdded->once() fails) then GREEN.
Drop deprecated setAccessible() call (no-op since PHP 8.1).

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

includes/class-plugin.php

@@ -6,6 +6,7 @@ namespace WPMultiCity;
 final class Plugin
 {
     private static ?Plugin $instance = null;
+    private bool $booted = false;
 
     public static function instance(): Plugin
     {
@@ -36,6 +37,11 @@ final class Plugin
 
     public function boot(): void
     {
+        if ($this->booted) {
+            return;
+        }
+        $this->booted = true;
+
         if (!$this->dependencies_satisfied()) {
             add_action('admin_notices', [$this, 'render_dependency_notice']);
             return;