Walk-forward backtest to detect overfitting — validates strategy on unseen data.

    Args:
        symbol: Yahoo Finance symbol (AAPL, BTC-USD, SPY…)
        strategy: rsi | bollinger | macd | ema_cross | supertrend | donchian
                  | keltner_breakout
                  (rsi_pullback and triple_ema not supported here — SMA200 warmup
                   exceeds typical fold size; use run_backtest with period='2y')
        period: '1mo', '3mo', '6mo', '1y', '2y' (recommend '2y')
        initial_capital: Starting capital per fold in USD (default $10,000)
        commission_pct: Per-trade commission % (default 0.1%)
        slippage_pct: Per-trade slippage % (default 0.05%)
        n_splits: Number of walk-forward folds (default 3, max 10)
        train_ratio: Fraction of each fold used for training (default 0.7)
        interval: '1d' (daily) or '1h' (hourly)