Draw the connected choice group without Tailwind

Plan step 36. <x-group> renders data-md-group with its size, variant,
shape, multiple and inline, a data-md-button-group row and
data-md-group-segment labels over its radios or checkboxes. group.css
draws the segments as M3's buttons with the toggle button's colours
from :has(:checked), the disabled treatment from :has(:disabled), the
focus ring from :has(:focus-visible) and the 48px target at xs and sm.

The connected shapes move into button-group.css (the corner scale,
square ends, the pressed and selected inner corners), which group.css
imports; <x-button-group> adopts them in its own rewrite. The group's
render tests move to GroupTest; AppBarTest (navigation group) follows
the new legend and row hooks of theme-toggle's rows.

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

tests/Feature/Components/AppBarTest.php

@@ -200,7 +200,7 @@ it('offers M3\'s three contrast levels, marking the one in force', function () {
         ->toContain('data-theme-toggle="contrast"')
         ->toContain('aria-label="Contrast"')
         // A connected button group over native radios, not the deprecated segmented button (N-04).
-        ->toContain('data-button-group="connected"')
+        ->toContain('data-md-button-group="connected"')
         ->toContain('name="material-contrast"')
         ->toContain('value="standard"')
         ->toContain('value="medium"')
@@ -216,7 +216,7 @@ it('names a theme row for a screen reader and legends it on request', function (
         ->not->toContain('<legend')
         ->and((string) $this->blade('<x-theme-toggle mode="picker" label="Appearance" />'))
         ->toContain('aria-label="Appearance"')
-        ->toContain('<legend class="mb-2 type-label-lg text-on-surface-variant">Appearance</legend>');
+        ->toContain('<legend data-md-group-legend>Appearance</legend>');
 });
 
 it('switches the theme through the store in three shapes', function () {