ExecutionEngine: PositionEffect Loop and Order Execution

[Muratich]

Depends on: #110
Design doc: docs/strategy_composable_engine_design.md (§1.4–1.5, Engine changes)

Engine runtime for the composable action system.
Does NOT implement @register_action handlers — Philip owns those in #110.

Scope:

• Add Portfolio.effects: list[PositionEffect] (internal SL/TP/trailing state)
• Expose API so buy action handler (Predicates, Rules, Action registry #110) can attach effects when
  stop_loss_pct / take_profit_pct are set on a buy
• Each bar, before strategy:
  1. Check PositionEffects → if triggered, close position
  2. strategy.on_candle() → Signal
  3. OrderExecutor.execute(signal) → update portfolio / trade log
• Legacy Python strategies unchanged (no effects, same bar loop)
• After one composable backtest: call calculate_metrics_from_trade_log()
  from src/analytics/ (same path as legacy)
• For each combo: run backtest → calculate_metrics_from_trade_log() → collect (params, MetricsReport); skip empty/failed runs (just do not repeat same calculations)

Out of scope:

• @register_action plugins (Predicates, Rules, Action registry #110)
• YAML compile / rules (Composable Strategy Foundation (YAML, compile, choices) #108–Predicates, Rules, Action registry #110)
• GridOptimizer (separate issue — create for Week 5 P0)
• Dashboard / API (Composable Strategies + Optimizer Best Params in Dashboard #113, #115)

Definition of Done:

• Bar loop order: effects → strategy → executor
• Unit tests: SL effect closes position, TP effect closes position
• Legacy strategies still backtest with no changes
• One composable backtest → trade log + MetricsReport
• Philip confirms portfolio.effects API works with buy handler