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

Collapse the multi-file plugin (buildQuery, controlPanel,
transformProps, index) into a single index.tsx. Contour delegates to
the shared buildSpatialQuery / transformSpatialProps from ../spatialUtils.
The Contour.tsx component, getSafeCellSize helper, and Multi.tsx's
getPoints import stay intact.

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