Complete event-driven backtest engine v1

[asoracca]

Changes

• Enforces close[t] signal generation and open[t+1] execution
• Adds injected DataFrames for network-free testing
• Implements target-weight sizing and cash reservation
• Adds order, fill, trade, position, cash, equity, and rejection ledgers
• Adds commissions and slippage sensitivity
• Adds a cost-matched buy-and-hold benchmark
• Adds methodology, design, and results documentation
• Adds 8 deterministic tests and GitHub Actions