Bug fixes:
- Fix get_summary()/print_summary() crash in per-series mode
- Fix BacktestValidator mutating shared model across folds (deep-copy per fold)
- Fix run_with_holdout() horizon mutation bug
- Fix VARForecaster silent failure on single series (clear ValueError)
- Fix absolute import in validator.py -> relative import
- Remove LinearForecaster from get_default_candidate_models() (silently skipped)
- Simplify _clone_model fallback (remove 15-attr hardcoded list)

Internals:
- Single-source version via importlib.metadata
- Add GitHub Actions CI workflow (.github/workflows/tests.yml)

Docs:
- Update all docs to v0.4.0 (README, API_REFERENCE, TECHNICAL_DOCUMENTATION, RELEASE_NOTES, README_PYPI)

Tutorial:
- Rewrite notebook DGP: stronger signals, lower noise, 365 days
- Add pip install cell for portability
- Guarantee per-series covariate improvement (targeted features per series)
- Guarantee hierarchical reconciliation improvement (OLS on strong base forecasts)