feat: add Monte Carlo portfolio simulation and bump to 0.3.1

- Add Monte Carlo forward simulation using Geometric Brownian Motion
- Support correlated multi-asset simulation with Cholesky decomposition
- Implement parallel execution via Rayon for performance
- Expose simulate_portfolio_mc function in Python bindings
- Update version from 0.3.0 to 0.3.1 across all project files

src/portfolio/mod.rs

@@ -2,8 +2,10 @@
 
 pub mod allocation;
 pub mod engine;
+pub mod monte_carlo;
 pub mod position;
 
 pub use allocation::{AllocationStrategy, CapitalAllocator};
 pub use engine::PortfolioEngine;
+pub use monte_carlo::{simulate_portfolio_forward, MonteCarloConfig, MonteCarloResult};
 pub use position::PositionManager;