WIREFRAME — welcome screen (Stage A · low-fi · pre-styling)
derived from: prototypes/welcome.json  (Screen > Card > Text, Text, Button)

+--------------------------------------------------+
|  SCREEN   [region: full viewport]                |
|                                                  |
|     +--------------------------------------+     |
|     |  CARD   [region: centered group]     |     |
|     |                                      |     |
|     |   [ Text slot ]    title  (bold)     |     |
|     |   [ Text slot ]    subtitle          |     |
|     |   [ Button slot ]  primary CTA       |     |
|     |                                      |     |
|     +--------------------------------------+     |
|                                                  |
+--------------------------------------------------+

slots map one-to-one to prototype elements:
  SCREEN  ->  Screen   (root)
  CARD    ->  Card     (welcome-card)
  Text    ->  Text     (title, subtitle)
  Button  ->  Button   (cta)

notes:
  - structure only — no color, no type scale, no spacing values yet
  - styling arrives in Stage B (kit.sample.css bound to tokens.sample.css)
  - confirm the STRUCTURE here is right before investing in the hi-fi mock
