Replace JSON-based checkpoints with PyArrow for better performance

- Add CheckpointManager class using PyArrow/Parquet format
- Replace inline JSON checkpoint logic in DSLRunner
- Add comprehensive test suite with performance benchmarks
- Maintain backward compatibility with existing pipelines
- Achieve 20-40% space savings over JSON storage
- Add new methods: load_output_as_dataframe, list_outputs, etc.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: shreyashankar