Tests/strategy

[folg-code]

Strategy layer refactor & tests

• Extracted a unified, deterministic TradePlan builder shared by live and backtest flows.
• Separated trade intent (strategy) from execution concerns (sizing, slippage, exits).
• Added comprehensive unit tests for strategy.
• Standardized plan validation, exit modes (fixed / managed) and level tag extraction for statistics.
• Reduced strategy responsibilities to signal generation only; execution logic moved to engines.