- Flatten package structure: move pkg/clickhouse to clickhouse, merge
  did and types sub-packages into the root cloudevent package
- Replace cloudevent_json.go with separate encoder/decoder files
- Add parquet encoder/decoder with full test coverage
- Add field_accessors.go for header field access
- Remove model-garage dependency and codegen
- Fix test imports and lint issues

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