Add LightGBM dtreeviz tree visualization support

[oegedijk]

Summary

• add LightGBM tree explainer support (LGBMExplainer) with dtreeviz shadow trees, decision-path rendering, and per-tree contribution plots
• wire LightGBM into explainer auto-detection and decision-tree dashboard components
• add a safe threaded dtreeviz backend fallback (matplotlib -> Agg) to prevent macOS callback crashes
• add/merge LightGBM treeviz regression tests into tests/test_boosting_models.py
• add a local demo launcher at scripts/run_lgbm_dashboard.py
• update README.md, docs/source/explainers.rst, docs/source/deployment.rst, and docs/source/index.rst to document LightGBM tree support
• update TODO.md and RELEASE_NOTES.md

Validation

• uv run pytest -q tests/test_boosting_models.py -k lgbm
• uv run pytest -q tests/test_xgboost_treeviz.py

Closes #118