* fix(catalog): token-themed component previews no longer render black
Seven components read theme tokens with no fallback, so with no theme
pack they painted dark text on a black canvas, both when installed and
in their live Catalog preview. They now carry exact-value fallbacks from
their own demos, as the theme contract requires, and a test fails any
registry item that reads a contract token bare.
The Catalog's snippet preview wrapper also copies the tokens each demo
sets on its mount, and paints the demo's background, so the preview
wears the demo's theme.
* fix(catalog): trim the preview theme helper comment
* test(catalog): strip HTML comments with the parser in the theme fallback gate
