feat(glyph): consolidate deckgl Screengrid layer to defineChart()

Collapse multi-file plugin into single index.tsx. Screengrid uses
shared buildSpatialQuery/transformSpatialProps + grid size + default/
fixed/categorical color scheme. Screengrid.tsx component stays as
sibling for Multi.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>