Link and Grid refinements

A larger Link size, consistent Grid spacing, and a clearer external-link API.

Added

Link now supports size="lg". Grid gains implicit track controls and border radius support for items.

Fixed

Grid gaps now use the same base unit as Stack, including axis-specific gaps and fallback values. Full-width links use a display mode that allows their width to apply.

Breaking changes

Rename the Link external prop to isExternal. Unsupported Interactive props that had no effect on Link have been removed from its public API.

<Link external href="https://example.com" />
<Link isExternal href="https://example.com" />