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/StochasticProcess.hpp

@@ -0,0 +1,32 @@
+/**
+ * @file StochasticProcess.hpp
+ * @brief Interface for SDE drift, diffusion, and time stepping.
+ */
+
+#ifndef QUANTENGINE_STOCHASTICPROCESS_HPP
+#define QUANTENGINE_STOCHASTICPROCESS_HPP
+#include "MarketData.hpp"
+#include <memory>
+
+/**
+ * @brief Stochastic model for the underlying, driven by @ref MarketData.
+ */
+class StochasticProcess {
+public:
+    StochasticProcess() = delete;
+    explicit StochasticProcess(MarketData data) : data_(std::move(data)){}
+
+    virtual ~StochasticProcess() = default;
+    virtual double drift(double t, double s) = 0;
+    virtual double diffusion(double t, double s) = 0;
+    virtual double step(double t, double s, double dt, double dW) = 0;
+    const MarketData& data() const {return data_;}
+
+
+private:
+    MarketData data_;
+
+};
+
+
+#endif //QUANTENGINE_STOCHASTICPROCESS_HPP