Restructure C++ core into cpp module and package bindings.

Move the pricing engine sources out of src/ into cpp/, add the closed-form engine and pybind wiring, and align tests/build targets with the new project layout.

Made-with: Cursor

cpp/MonteCarloEngine.hpp

@@ -0,0 +1,26 @@
+/**
+ * @file MonteCarloEngine.hpp
+ * @brief Monte Carlo pricing using a @ref StochasticProcess and @ref RandomGenerator.
+ */
+
+#ifndef QUANTENGINE_MONTECARLOENGINE_HPP
+#define QUANTENGINE_MONTECARLOENGINE_HPP
+#include "PricingEngine.hpp"
+#include "RandomGenerator.hpp"
+
+/**
+ * @brief Simple path simulation: one Euler/exact step to horizon, average discounted payoff.
+ */
+class MonteCarloEngine : public PricingEngine{
+public:
+    MonteCarloEngine() = default;
+    MonteCarloEngine(int numPaths, std::unique_ptr<StochasticProcess> process, std::shared_ptr<RandomGenerator> rng):
+    numPaths_(numPaths), PricingEngine(std::move(process)), rng_(std::move(rng)) {}
+    double calculate(const Instrument& instrument) const override;
+private:
+    int numPaths_;
+    std::shared_ptr<RandomGenerator> rng_;
+};
+
+
+#endif //QUANTENGINE_MONTECARLOENGINE_HPP