Propagate buy_signal into backtest records and trade analysis

[yafeng2003]

Motivation

• Make it possible to attribute trades and P/L to originating signals by recording the buy signal throughout the backtest lifecycle.
• Enable per-signal breakdowns in the trade analysis CSV and console report for deeper quantitative decomposition of strategy behavior.

Description

• Added an optional buy_signal parameter to BacktestStrategy.upgrade_records and store buy_signal in each record written to self.records via the buy_signal field.
• When pending orders are filled, pass the originating buy_signal into upgrade_records, and propagate the stored buy_signal for HOLD and SELL records by extracting it from self.holdings where appropriate.
• In BacktestResultCalculator.analyze_transactions, capture buy_signal from BUY rows into active_positions and include buy_signal in each trade entry produced by _add_trade_record so the analysis CSV contains signal attribution.

Testing

• No automated tests were executed for this change (analysis-only change).

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Codex Task