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/VolatilitySurface.hpp

@@ -0,0 +1,28 @@
+/**
+ * @file VolatilitySurface.hpp
+ * @brief Implied volatility as a function of strike and expiry.
+ */
+
+#ifndef QUANTENGINE_VOLATILITYSURFACE_HPP
+#define QUANTENGINE_VOLATILITYSURFACE_HPP
+#include <vector>
+
+/**
+ * @brief Local/vol surface @f$\sigma(K,T)@f$ used by simulation.
+ */
+class VolatilitySurface {
+public:
+    virtual ~VolatilitySurface() = default;
+    virtual double sigma(double K, double T) const = 0;
+private:
+
+};
+
+class SVI : public VolatilitySurface {
+public:
+    SVI() = default;
+    SVI(std::vector<double> K, std::vector<double> rho, std::vector<double> S, std::vector<double> T);
+};
+
+
+#endif //QUANTENGINE_VOLATILITYSURFACE_HPP