Say when to use each token on the showcase's foundation pages, and name both skills in the README

The paragraphs restate the material-3 guideline's rules beside the tokens they
govern (plan step 28); the README's AI agents section names the second guideline
and skill (step 30).

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

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

@@ -5,6 +5,10 @@
         The corner scale as <code>rounded-corner-*</code>, and M3 Expressive's shapes as <code>&lt;x-shape name="…" /&gt;</code>.
     </p>
 
+    <p class="max-w-3xl type-body-md text-on-surface-variant" data-test="when-to-use">
+        When to use which: <code>full</code> for buttons, icon buttons, chips' avatars, badges, switches, sliders, the search bar and navigation indicators; <code>xs</code> for text fields, menus, snackbars and plain tooltips; <code>sm</code> for chips; <code>md</code> for cards and rich tooltips; <code>lg</code> for the FAB and a side sheet's inner corners; <code>xl</code> for dialogs, bottom sheets, pickers and carousel items; <code>xxl</code> for hero containers. A nested corner is the outer radius minus the padding. A press squares a round shape; nothing morphs on hover. The shapes below are decoration, never meaning.
+    </p>
+
     <div class="grid grid-cols-2 gap-4 medium:grid-cols-5">
         @foreach (['rounded-corner-none', 'rounded-corner-xs', 'rounded-corner-sm', 'rounded-corner-md', 'rounded-corner-lg', 'rounded-corner-lg-increased', 'rounded-corner-xl', 'rounded-corner-xl-increased', 'rounded-corner-xxl', 'rounded-corner-full'] as $corner)
             <div class="space-y-2">