Draw the menu group without Tailwind

<x-menu-group> renders data-md-menu-group with data-md-gap and no
class list; menu-group.css draws the plain cluster's 4px padding and
the gapped Expressive layout's 2px item gap and 8px cluster margin
from SegmentedMenuTokens (plan step 36, actions).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

tests/Feature/Components/ActionStylesheetsTest.php

@@ -20,6 +20,7 @@ dataset('action components', [
     'split-button',
     'fab',
     'menu-separator',
+    'menu-group',
 ]);
 
 it('draws the component from a stylesheet shaped like every package stylesheet', function (string $name) {