Repair Rust portfolio module placement/integration and restore portfolio Monte Carlo build path

[Copilot]

mc_portfolio.rs was not loadable by the crate due to a malformed filesystem path, so portfolio Monte Carlo logic (asset/scenario structs, correlation/draw/aggregation pipeline, VaR/CVaR, diversification metrics) existed but was effectively disconnected from the build. This PR restores a valid module layout and wires portfolio functionality into the Rust crate/PyO3 surface.

• Module restoration

  • Moves portfolio implementation into a valid Rust source file path: data-engine/rust/src/mc_portfolio.rs.
  • Removes malformed newline/tree-character path artifacts that prevented module resolution.

• Crate integration

  • Adds mod mc_portfolio; and imports required portfolio types/functions in lib.rs.
  • Exposes portfolio entry points through PyO3 so portfolio simulation and stress APIs are reachable from Python.

• Type and API alignment

  • Ensures portfolio scenario typing is consistent (PortfolioScenario/result structs) and matches call sites.
  • Normalizes naming and references for portfolio VaR/CVaR and diversification outputs.

• Dependency/module consistency fixes

  • Aligns module naming (distribution vs distributions) and dependency declarations needed by the restored portfolio path (serde/serde_json, RNG feature compatibility).

// lib.rs
mod mc_portfolio;

use mc_portfolio::{PortfolioAsset, run_portfolio, run_single_portfolio_scenario};