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/components/checkbox.blade.php

@@ -10,7 +10,13 @@
      (resources/css/components/selection.css).
 
      Without a label — a "select all" in a table header, a row's tick — the row is only the 18px box,
-     so the box carries `touch-target` and catches presses over M3's 48px minimum. --}}
+     so the box carries `touch-target` and catches presses over M3's 48px minimum.
+
+     Grouping is the caller's, not this component's: M3 asks that from `expanded` (840px) a set of
+     related checkboxes be gathered into a contained region rather than left as one long column
+     (docs/reference/m3/components-navigation-selection-inputs.md § Checkbox, Behaviour). Lay the
+     group out yourself — `<div class="grid gap-4 expanded:grid-cols-2">` around the checkboxes, or a
+     `<x-card>` or side sheet holding them — and give the group a heading that names what it asks. --}}
 
 @props([
     'label' => null,