Theme preview example

Composition sketch for Brickwork Theme Phase D preview surfaces.

• Load pack tokens.css after brickwork CSS.
• Set data-bw-brand (and data-density for L4 when claimed).
• Render shell + card + form + one marketing band + {% bw_token_specimen level="L3" %} (or L4).

See THEME.md (Preview surfaces) and brand-pack.md.