docs: Add Phase 7 implementation plan with synthetic data spec

- Add comprehensive Phase 7 (Sliding Window Regression) implementation plan
- Include M7.1, M7.2, M7.3 milestone specifications
- Add Section D7.1.4: Realistic synthetic TPC distortion data specification
- Update restartContext with statsmodels decision and M7.1 scope
- Document zero-copy accumulator architecture (MEM-3)
- Specify statsmodels integration for multiple fitter types (OLS, WLS, GLM, RLM)

Plan reviewed and approved by:
- GPT-4: Architecture and API design
- Gemini: Physical model and algorithm correctness
- MI: Statsmodels integration decision (2025-10-27)

Ready for M7.1 implementation via test-driven development approach.

Author: miranov25