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>

tests/PluginBootTest.php

@@ -29,4 +29,38 @@ final class PluginBootTest extends TestCase
             'boot() must register FieldGroup on acf/init'
         );
     }
+
+    public function test_boot_is_idempotent_when_called_multiple_times(): void
+    {
+        if (!class_exists('ACF', false)) {
+            eval('class ACF {}');
+        }
+
+        // Reset the singleton's boot flag so this test is independent of
+        // whatever previous tests did to the static instance. (We will add
+        // the flag in GREEN.)
+        $plugin = Plugin::instance();
+        $reflection = new \ReflectionClass($plugin);
+        if ($reflection->hasProperty('booted')) {
+            $prop = $reflection->getProperty('booted');
+            $prop->setValue($plugin, false);
+        }
+
+        \Brain\Monkey\Actions\expectAdded('init')
+            ->with([\WPMultiCity\Taxonomy::class, 'do_register'])
+            ->once();
+        \Brain\Monkey\Actions\expectAdded('init')
+            ->with([\WPMultiCity\PostType::class, 'do_register'])
+            ->once();
+        \Brain\Monkey\Actions\expectAdded('acf/init')
+            ->with([\WPMultiCity\FieldGroup::class, 'do_register'])
+            ->once();
+
+        $plugin->boot();
+        $plugin->boot();
+        $plugin->boot();
+
+        // Mockery verifies ->once() in tearDown.
+        self::assertTrue(true);
+    }
 }