feat(portfolio): add Portfolio State for holdings and mark-to-market - Issue #29 (68 tests)

Implements comprehensive portfolio state management:
- Holding dataclass with long/short support and P&L calculations
- CashBalance for multi-currency cash management
- PortfolioState class with:
  - Real-time mark-to-market valuation
  - Multi-currency support with exchange rate conversion
  - Thread-safe state updates
  - Position tracking with average cost calculation
  - Portfolio snapshots for historical tracking
- PriceProvider and ExchangeRateProvider protocols
- Serialization/deserialization support

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