feat: implement evolution engine for self-improving strategies

Complete Pillar 4 implementation with comprehensive testing and analysis.

Components:
- EvolutionOptimizer: Analyzes losing decisions from DecisionLogger,
  identifies failure patterns (time, market, action), and uses Gemini
  to generate improved strategies with auto-deployment capability
- ABTester: A/B testing framework with statistical significance testing
  (two-sample t-test), performance comparison, and deployment criteria
  (>60% win rate, >20 trades minimum)
- PerformanceTracker: Tracks strategy win rates, monitors improvement
  trends over time, generates comprehensive dashboards with daily/weekly
  metrics and trend analysis

Key Features:
- Uses DecisionLogger.get_losing_decisions() for failure identification
- Pattern analysis: market distribution, action types, time-of-day patterns
- Gemini integration for AI-powered strategy generation
- Statistical validation using scipy.stats.ttest_ind
- Sharpe ratio calculation for risk-adjusted returns
- Auto-deploy strategies meeting 60% win rate threshold
- Performance dashboard with JSON export capability

Testing:
- 24 comprehensive tests covering all evolution components
- 90% coverage of evolution module (304 lines, 31 missed)
- Integration tests for full evolution pipeline
- All 105 project tests passing with 72% overall coverage

Dependencies:
- Added scipy>=1.11,<2 for statistical analysis

Closes #19

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/evolution/__init__.py

@@ -0,0 +1,19 @@
+"""Evolution engine for self-improving trading strategies."""
+
+from src.evolution.ab_test import ABTester, ABTestResult, StrategyPerformance
+from src.evolution.optimizer import EvolutionOptimizer
+from src.evolution.performance_tracker import (
+    PerformanceDashboard,
+    PerformanceTracker,
+    StrategyMetrics,
+)
+
+__all__ = [
+    "EvolutionOptimizer",
+    "ABTester",
+    "ABTestResult",
+    "StrategyPerformance",
+    "PerformanceTracker",
+    "PerformanceDashboard",
+    "StrategyMetrics",
+]