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

Collapse multi-file plugin into single index.tsx. Scatter keeps its
custom buildQuery (spatial config required, metric-or-fixed radius
handling) and transformProps (radius from metric/fix value, category
column, js columns).

Also wires `onInit?: ControlPanelConfig['onInit']` through to
defineChart so Scatter can clear time_grain_sqla and granularity on
chart initialization. The onInit hook is now an optional metadata-level
override surfaced in the generated ControlPanelConfig.

Scatter.tsx component stays as sibling for Multi.

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