Exposes ensemble feature importance as a standalone endpoint per R5.1 spec.
Aggregates XGBoost (0.4) + LightGBM (0.35) + CatBoost (0.25) gain when trained
boosters are loaded; falls back to the curated heuristic ranking otherwise, so
callers can depend on the endpoint during scaffold/heuristic-only runs.
- Factored heuristic drivers into a shared constant (_HEURISTIC_DRIVERS)
- Added AVMv2FeatureImportanceResponse model (model_version + source + drivers)
- Added service.get_feature_importance() public method
- Added tests/test_avm_v2.py::test_feature_importance_heuristic (24 total pass)
Co-Authored-By: Paperclip <noreply@paperclip.ing>
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