Draw the button group without Tailwind

Plan step 36. <x-button-group> renders data-md-button-group (standard or
connected), data-md-size, data-md-shape and, with a selection,
data-md-selection and data-md-selection-required. button-group.css now
also draws the standard group: its gaps per size, the square corner scale
it hands its buttons, and the press expansion that widens a pressed label
button while its neighbours give way. groups.css keeps only the split
button until that is rewritten too.

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

tests/Feature/Components/ActionStylesheetsTest.php

@@ -16,6 +16,7 @@ dataset('action components', [
     'badge',
     'button',
     'group',
+    'button-group',
 ]);
 
 it('draws the component from a stylesheet shaped like every package stylesheet', function (string $name) {