Add atr_sl_mult and atr_tp_mult parameters to _calc_labels_vectorized
and generate_dataset_vectorized, defaulting to existing constants (1.5,
2.0) for full backward compatibility. Callers (train scripts, backtester)
can now pass symbol-specific multipliers without modifying module-level
constants.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>