Add DATA_PIPELINE_ANALYSIS.md from codebase exploration

This document was generated during the comprehensive codebase review
and contains detailed analysis of the data pipeline:

- Dataset implementations (Volume, Tile, Filename, Multi)
- Augmentation system (1449 lines of MONAI transforms)
- Transform builders (791 lines with train/val/test pipelines)
- I/O utilities (452 lines, multi-format support)
- Configuration system
- Code quality assessment (7.5/10)
- Identified issues and recommendations

Key findings:
- 9,373 lines total in data module
- Well-structured with MONAI-first approach
- 3 high-priority issues (NotImplementedError, import cycle, dummy dataset)
- 6 medium-priority improvements (code duplication, hardcoded values)
- Overall production-ready with minor refactoring recommended

This complements the main REFACTORING_PLAN.md document.

Author: claude