Show M3's window size classes on a Layout page of the showcase

The page names the class this window is in, lights the five cards through the
compiled variants themselves, and tabulates what each class changes: navigation,
panes, dialogs and sheets, margins. Plan step 28, in part.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

src/Showcase/Sections.php

@@ -87,6 +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.'],
             '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.'],