box: side-only borders render nothing without an explicit height

Summary

A box with a side-only border and no children does not draw its border unless height={1} is set explicitly. The rule is silently absent, which is easy to misread as a theming problem.

Environment

• @opentui/core 0.5.11 (via @opentui/solid 0.5.11)
• Bun 1.4.0, Linux

Steps to reproduce

1. <box border={["top"]} borderColor="#505050" /> (no height).
2. Compare with <box border={["top"]} height={1} borderColor="#505050" />.

Actual

Without an explicit height the box collapses and the top border is not visible.

Expected

Either auto-size a side-only border to at least 1 row, or document that side-only borders require an explicit height (a dev warning would also help).

Workaround

Set height={1}.