Ease a collapse open, which its own comment already promised

Plan step 19, containment.md C-24. `interpolate-size: allow-keywords`
was set and nothing transitioned, so the content snapped open. A new
resources/css/components/collapse.css transitions `block-size` on
`::details-content` over the fast spatial spring, with
`content-visibility` `allow-discrete` beside it so the section stays
rendered while it closes. The `<details>` carries `data-collapse` for
the rule to find.

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

tests/Feature/Components/CollapseTest.php

@@ -11,6 +11,8 @@ it('discloses on the native details element, kept open through a morph', functio
         ->toContain('wire:ignore.self')
         ->toContain(' open ')
         ->toContain('rounded-corner-lg bg-surface-container')
+        ->toContain('data-collapse')
+        ->toContain('[interpolate-size:allow-keywords]')
         ->toContain('How long do links last?')
         ->toContain('Until the expiry.')
         ->toContain('group-open/collapse:rotate-180');
@@ -20,12 +22,12 @@ it('has no Alpine on it without a binding', function () {
     $html = (string) $this->blade('<x-collapse title="Advanced" open>Body</x-collapse>');
 
     expect($html)
-        ->toMatch('/<details\s+wire:ignore.self\s+open\s+class="group\/collapse/')
+        ->toMatch('/<details\s+wire:ignore.self\s+data-collapse\s+open\s+class="group\/collapse/')
         ->not->toContain('x-data')
         ->not->toContain('x-modelable')
         ->not->toContain('x-on:toggle')
         ->and((string) $this->blade('<x-collapse title="Advanced">Body</x-collapse>'))
-        ->toMatch('/<details\s+wire:ignore.self\s+class="group\/collapse/');
+        ->toMatch('/<details\s+wire:ignore.self\s+data-collapse\s+class="group\/collapse/');
 });
 
 it('binds its open state through x-model, drawing the open prop until Alpine starts', function () {