feat(glyph): consolidate plugin-chart-word-cloud to defineChart()

Collapse the multi-file plugin/ subdirectory (buildQuery, controlPanel,
transformProps, index) into a single top-level src/index.tsx using
defineChart(). The chart/WordCloud.tsx component stays as a sibling.

  - Move plugin/controls/ up to src/controls/ so the new index.tsx can
    import the local RotationControl and ColorSchemeControl directly.
  - Delete src/index.ts barrel (the new index.tsx is the entrypoint and
    re-exports from ./types).
  - WordCloudChartPlugin is exported both as default and named, matching
    the previous public API.

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