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/FlatVolatilitySurface.hpp

@@ -0,0 +1,21 @@
+/**
+ * @file FlatVolatilitySurface.hpp
+ * @brief Constant implied volatility surface.
+ */
+#ifndef QUANTENGINE_FLATVOLATILITYSURFACE_HPP
+#define QUANTENGINE_FLATVOLATILITYSURFACE_HPP
+#include "VolatilitySurface.hpp"
+
+/**
+ * @brief @f$\sigma(K,T)\equiv\sigma_0@f$.
+ */
+class FlatVolatilitySurface : public VolatilitySurface {
+public:
+    explicit FlatVolatilitySurface(double sigma = 0.2) : sigma_(sigma) {}
+
+    double sigma(double K, double T) const override {return sigma_;}
+
+private:
+    double sigma_;
+};
+#endif