arena.mjs: fix up arrow button by adding missing box import

The up button buffer was missing 'box' in its destructuring (line 452),
but the code tries to use box() on lines 463-464 to draw decorative dots.
This caused the decorative dots to be undefined and not render.

The down, left, and right buttons all properly imported box, so only the
up button was missing it.

- Added 'box' to the destructuring: const { wipe, ink, line, box } = api;
- Allows decorative dots to render properly on the up arrow button

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>