Say how M3 wants a checkbox group laid out at expanded

M3 § Checkbox, Behaviour asks that from expanded (840px) related checkboxes be
gathered into a contained region rather than left as one long column. That is a
rule about the page around the control, not about the control, so it is written
down rather than built: the component's header and its SKILL.md entry now say
it, and the showcase lays a group out in an `expanded:grid-cols-2` card (plan
step 24, audit docs/audits/m3-alignment/inputs.md § Missing).

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

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

@@ -92,6 +92,23 @@
                 </div>
             </div>
             BLADE,
+        'A checkbox group at expanded' => <<<'BLADE'
+            {{-- M3: from expanded (840px), gather related checkboxes into a contained region instead of one long column. --}}
+            <x-card variant="outlined" class="w-full max-w-2xl">
+                <fieldset>
+                    <legend class="mb-4 type-title-sm text-on-surface">Tell me about</legend>
+
+                    <div class="grid gap-4 expanded:grid-cols-2">
+                        <x-checkbox label="Downloads" name="notify[]" value="downloads" checked />
+                        <x-checkbox label="Comments" name="notify[]" value="comments" />
+                        <x-checkbox label="Expiring shares" name="notify[]" value="expiring" checked />
+                        <x-checkbox label="Failed uploads" name="notify[]" value="failed" />
+                        <x-checkbox label="New team members" name="notify[]" value="members" />
+                        <x-checkbox label="Weekly summary" name="notify[]" value="summary" />
+                    </div>
+                </fieldset>
+            </x-card>
+            BLADE,
         'Radio buttons' => <<<'BLADE'
             <div class="grid w-full gap-6 medium:grid-cols-2">
                 <x-radio label="Who can open the link" name="showcase-audience" value="password" :options="[['id' => 'anyone', 'name' => 'Anyone with the link'], ['id' => 'password', 'name' => 'Anyone with the password', 'hint' => 'Share the password separately'], ['id' => 'team', 'name' => 'My team only', 'disabled' => true]]" />