Tie a collapse's padding and chevron to its own parts

Plan step 36 review of 76abdd62. The old view put the filled variant's
16px padding on its own summary and body; the stylesheet matched them as
descendants of any filled collapse, so a plain collapse nested inside
one took the padding too (and an open one turned every nested chevron).
Child combinators now tie each rule to its own <details>. The 12px gap is
a literal like alert.css's, not an average of two spacing tokens.

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

tests/Feature/Components/CollapseTest.php

@@ -2,6 +2,7 @@
 
 use Livewire\Component;
 use Livewire\Livewire;
+use NoNameWeb\LivewireMaterial\Tests\Support\ComponentStylesheet;
 
 it('discloses on the native details element, kept open through a morph', function () {
     $html = (string) $this->blade('<x-collapse title="How long do links last?" icon="schedule" open variant="filled">Until the expiry.</x-collapse>');
@@ -65,3 +66,12 @@ it('entangles its open state with a Livewire property and renders open as the pr
 
     expect(preg_match('/\sopen\s/', $tag[0]) === 1)->toBe($fineTuning);
 })->with(['open' => true, 'closed' => false]);
+
+it('draws a filled collapse\'s padding and an open one\'s chevron on its own parts, never a nested collapse\'s', function () {
+    $css = ComponentStylesheet::read('collapse');
+
+    expect($css->declarations("[data-md-collapse][data-md-variant='filled'] > [data-md-collapse-summary]"))->toBe(['padding-inline' => 'var(--md-sys-measurement-space200)'])
+        ->and($css->declarations("[data-md-collapse][data-md-variant='filled'] > [data-md-collapse-body]"))->toBe(['padding-inline' => 'var(--md-sys-measurement-space200)'])
+        ->and($css->declarations('[data-md-collapse][open] > [data-md-collapse-summary] > [data-md-collapse-chevron]'))->toBe(['rotate' => '180deg'])
+        ->and($css->declarations('[data-md-collapse-summary]'))->toHaveKey('gap', '12px');
+});