Add the pane layout component

Plan step 35: <x-pane>, a content region with M3's margins (16px below
medium, 24px from it) drawn once however panes and layouts nest, a width
cap, and an optional pane app bar - <x-app-bar> as a direct child of the
pane with title, subtitle, actions, a back link or action, a leading
slot, and the section navigation under it.

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

resources/views/showcase/sections/layout.blade.php

@@ -113,6 +113,7 @@
         </p>
 
         <x-livewire-material::stack as="ul" gap="space100" class="md-type-body-md">
+            <li><code>&lt;x-pane&gt;</code> — a content region with M3's margins and its own app bar.</li>
             <li><code>&lt;x-surface&gt;</code> — a tonal region: a surface role, padding, a corner and an outline.</li>
             <li><code>&lt;x-stack&gt;</code>, <code>&lt;x-row&gt;</code> and <code>&lt;x-grid&gt;</code> — arrangement inside a pane.</li>
         </x-livewire-material::stack>