Rename the app shell to the scaffold and give it a FAB slot

Plan step 35: <x-app-shell> becomes <x-scaffold>, M3's and Compose's name
for the structure of bars, rails and panes, with no alias; every package
view, the showcase, the tests, the skills, README and UPGRADE follow. Its
markup and Tailwind classes stay for the navigation rewrite. The new fab
slot places the FAB as Compose's Scaffold does - bottom-end, 16px from the
edges below medium and 24px from it, clear of the navigation bar and of a
snackbar on screen (resources/css/layout/scaffold.css) - and the content
region tells a pane inside that the margin is already drawn.

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

@@ -98,7 +98,7 @@
     </div>
 
     <p class="max-w-3xl type-body-md text-on-surface-variant">
-        <code>&lt;x-app-shell&gt;</code> does all of this: resize this showcase and watch the bar, the rail and the
+        <code>&lt;x-scaffold&gt;</code> does all of this: resize this showcase and watch the bar, the rail and the
         content margin (<code>--material-margin</code>) change at 600, 840 and 1200px. A visitor who has chosen a rail
         width keeps it; until then the rail follows the class.
     </p>
@@ -113,6 +113,7 @@
         </p>
 
         <x-livewire-material::stack as="ul" gap="space100" class="md-type-body-md">
+            <li><code>&lt;x-scaffold&gt;</code> — the bars, the rail and the FAB around the page (this showcase is one).</li>
             <li><code>&lt;x-pane&gt;</code> — a content region with M3's margins and its own app bar.</li>
             <li><code>&lt;x-list-detail&gt;</code> — a list and the detail of its selection: one pane below 840px, two from it.</li>
             <li><code>&lt;x-supporting-pane&gt;</code> — a focus pane and a pane beside it from 840px, below it or a bottom sheet before.</li>