Fix GridCell area placement

[vaporwavie]

Summary

Fix GridCell named-area placement so area is not overridden by implicit
auto placement values.

• Stop emitting grid placement CSS variables when the related prop was not
  provided.
• Remove the default --sl-grid-cell-* declarations from the grid cell theme
  CSS, avoiding cascade resets of grid-area.
• Add a regression test covering named-area placement and explicit placement
  overrides.

Closes #1880.

Examples

<Grid
  columns="1fr 1fr"
  areas={`
    "header header"
    "main aside"
  `}
>
  <GridCell area="header">Header</GridCell>
  <GridCell area="main">Main</GridCell>
  <GridCell area="aside">Aside</GridCell>
</Grid>

[vercel]

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
shoreline-docs	Ignored		Mar 26, 2026 6:04pm