refactor(common): delete MLFeatureExtractor + SimpleDQNAdapter, refactor SharedMLStrategy
- Delete MLFeatureExtractor (1,294 lines) and SimpleDQNAdapter (235 lines)
- Delete 830 lines of inline tests for deleted types
- Remove legacy_feature_extractor field from SharedMLStrategy
- Replace new() and new_with_production_extractor() with new(extractor, models, threshold)
- Single constructor accepts injected models via Vec<Box<dyn MLModelAdapter>>
- Update all callers: backtesting_service, 2 integration tests, 2 trading_service tests
- Fix doc comments referencing MLFeatureExtractor
- Fix feature count test: real extractor produces 51 features, not 225
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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