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

src/Showcase/Sections.php

@@ -87,7 +87,7 @@ class Sections
             'shape' => ['title' => 'Shape', 'icon' => 'interests', 'group' => 'Foundations', 'description' => 'Corner radii and the 35 M3 Expressive shapes.'],
             'elevation' => ['title' => 'Elevation', 'icon' => 'layers', 'group' => 'Foundations', 'description' => 'Shadows for what floats over content.'],
             'motion' => ['title' => 'Motion', 'icon' => 'animation', 'group' => 'Foundations', 'description' => 'Spatial springs and effects easings.'],
-            'layout' => ['title' => 'Layout', 'icon' => 'devices', 'group' => 'Foundations', 'description' => 'M3\'s window size classes, what changes at each, and the app shell\'s response.'],
+            'layout' => ['title' => 'Layout', 'icon' => 'devices', 'group' => 'Foundations', 'description' => 'M3\'s window size classes, what changes at each, and the scaffold\'s response.'],
             'icons' => ['title' => 'Icons', 'icon' => 'emoji_symbols', 'group' => 'Foundations', 'description' => 'Every Material Symbol, searchable, outlined and filled.'],
             'buttons' => ['title' => 'Buttons', 'icon' => 'smart_button', 'group' => 'Actions', 'description' => 'Buttons, icon buttons, groups, split buttons and FABs.'],
             'menus' => ['title' => 'Menus', 'icon' => 'menu_open', 'group' => 'Actions', 'description' => 'Menus with items, groups, choices and shortcuts.'],
@@ -101,7 +101,7 @@ class Sections
             'pickers' => ['title' => 'Date pickers', 'icon' => 'calendar_month', 'group' => 'Inputs', 'description' => 'Docked, modal and input date pickers, single and range.'],
             'timepickers' => ['title' => 'Time pickers', 'icon' => 'schedule', 'group' => 'Inputs', 'description' => 'The dial and input time picker.'],
             'bars' => ['title' => 'App bars and tabs', 'icon' => 'toolbar', 'group' => 'Navigation', 'description' => 'Top app bars, toolbars, tabs, section navigation and the account menu.'],
-            'navigation' => ['title' => 'Navigation', 'icon' => 'explore', 'group' => 'Navigation', 'description' => 'The navigation bar, the navigation rail and the app shell.'],
+            'navigation' => ['title' => 'Navigation', 'icon' => 'explore', 'group' => 'Navigation', 'description' => 'The navigation bar, the navigation rail and the scaffold.'],
             'data' => ['title' => 'Data', 'icon' => 'table_chart', 'group' => 'Data and pages', 'description' => 'Data tables, sort headers and pagination.'],
             'pages' => ['title' => 'Error pages and mail', 'icon' => 'page_info', 'group' => 'Data and pages', 'description' => 'The HTTP error pages and the Markdown mail theme.'],
         ];