Drop the old data-dragged hook from md-state-layer

Plan step 36 review. interaction.css read `data-dragged` beside
`data-md-dragged` while <x-card> still set the old name; the card now
sets the new one and nothing in the package renders the old one on an
element with the class, which 2.0.0 introduces. The Tailwind
`state-layer` utility keeps `data-dragged` until step 39 deletes it.
UPGRADE.md's card line named the old `data-card` hook.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

UPGRADE.md

@@ -84,7 +84,7 @@ over a colour, use the role directly.
   edge-to-edge items; items, not the row, are the tab stops.
 - `<x-list dividers>` draws a 16px-inset rule; `<x-list selectable>` or `selection="single|multi"`
   makes it a `listbox` of `option`s with `aria-selected`.
-- `<x-card>` changes elevation on hover instead of its corner; `data-card` carries the variant.
+- `<x-card>` changes elevation on hover instead of its corner; `data-md-card` carries the variant.
 - `<x-modal>` pins its header and actions and scrolls only the body; a `box-class` that set
   `overflow` no longer applies.
 - `<x-table>` rows are 52px and the automatic fine-pointer density is gone: pass `dense` for 36px