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/PricingEngine.hpp

@@ -0,0 +1,30 @@
+/**
+ * @file PricingEngine.hpp
+ * @brief Abstract pricer for @ref Instrument given a stochastic model.
+ */
+
+#ifndef QUANTENGINE_PRICINGENGINE_HPP
+#define QUANTENGINE_PRICINGENGINE_HPP
+#include <memory>
+
+#include "StochasticProcess.hpp"
+
+class Instrument;
+
+/**
+ * @brief Computes model price of an instrument (e.g. Monte Carlo, PDE, closed form).
+ */
+class PricingEngine {
+public:
+    PricingEngine() = default;
+    PricingEngine(std::unique_ptr<StochasticProcess> process) : process_(std::move(process)){}
+
+    virtual ~PricingEngine() = default;
+    virtual double calculate(const Instrument& instrument) const = 0;
+protected:
+    std::unique_ptr<StochasticProcess> process_;
+
+};
+
+
+#endif //QUANTENGINE_PRICINGENGINE_HPP