⚡ Optimize trading simulation performance (~640x speedup)

[EiJackGH]

• Replaced iterrows() loop with iteration over NumPy arrays and list accumulation in simulate_trading, achieving ~640x speedup (8.27s -> 0.013s for 10k days).
• Refactored CLI logic into a main() function to improve modularity and testability.
• Removed redundant/broken if __name__ == "__main__": block.
• Ensured PEP 8 compliance.

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PR created automatically by Jules for task 14119623549153839129 started by @EiJackGH

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