Plan step 36 (containment group): <x-collapse>'s Tailwind-era
resources/css/components/collapse.css (the height animation, C-24)
becomes the plain stylesheet, keyed on data-md-collapse and
data-md-variant; the summary renders the shared md-state-layer and
md-focus-ring classes instead of copying their rules. Leaves
tailwind.css for the Containment block of components.css.
Icon-to-title and title-to-chevron gaps stay 12px (not an M3 number;
collapse isn't an M3 component), written as the average of the two
adjacent spacing steps since the scale has no exact token for it.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9
