feat: strategy parameter sweep and production param optimization

- Add independent backtest engine (backtester.py) with walk-forward support
- Add backtest sanity check validator (backtest_validator.py)
- Add CLI tools: run_backtest.py, strategy_sweep.py (with --combined mode)
- Fix train-serve skew: unify feature z-score normalization (ml_features.py)
- Add strategy params (SL/TP ATR mult, ADX filter, volume multiplier) to
  config.py, indicators.py, dataset_builder.py, bot.py, backtester.py
- Fix WalkForwardBacktester not propagating strategy params to test folds
- Update production defaults: SL=2.0x, TP=2.0x, ADX=25, Vol=2.5
  (3-symbol combined PF: 0.71 → 1.24, MDD: 65.9% → 17.1%)
- Retrain ML models with new strategy parameters

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

models/xrpusdt/training_log.json

@@ -23,5 +23,80 @@
       "reg_lambda": 0.000157
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+    "model_path": "models/xrpusdt/lgbm_filter.pkl",
+    "tuned_params_path": null,
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+  },
+  {
+    "date": "2026-03-06T22:37:08.529734",
+    "backend": "lgbm",
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+    "model_path": "models/xrpusdt/lgbm_filter.pkl",
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+    "lgbm_params": {
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+  {
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   }
 ]