feat(portfolio): add Performance Metrics with Sharpe, drawdown, returns - Issue #31 (63 tests)
Implements comprehensive portfolio performance analytics:
- Returns calculation (daily, monthly, yearly, cumulative)
- Risk-adjusted metrics (Sharpe, Sortino, Calmar ratios)
- Volatility and downside deviation
- Drawdown analysis with period tracking
- Trade statistics (win rate, profit factor, expectancy)
- Benchmark comparison (alpha, beta, information ratio, tracking error)
- Utility functions (CAGR, rolling returns, period aggregation)
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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