Phase 2: V3/V4 enhancements - MAD diagnostic added

Changes:
- Fixed import paths in V3/V4 tests (17 test failures resolved)
- Added MAD (Median Absolute Deviation) to V3 and V4
- MAD computed from unweighted residuals for robustness
- Parameter errors working in V3 and V4

Status:
- 39/40 tests passing (97.5%)
- Known issue: test_v3_v4_parity fails (column mismatch)
- Will be fixed in next commit (standardization)

Production use:
- Code already deployed in ALICE TPC studies
- RMS and MAD used for quality assessment
- Outlier detection working

Next step: Output standardization (suffix on diagnostics)

Author: miranov25