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/FlatYieldCurve.hpp

@@ -0,0 +1,22 @@
+/**
+ * @file FlatYieldCurve.hpp
+ * @brief Constant zero rate yield curve.
+ */
+#ifndef QUANTENGINE_FLATYIELDCURVE_HPP
+#define QUANTENGINE_FLATYIELDCURVE_HPP
+#include "YieldCurve.hpp"
+#include <cmath>
+
+/**
+ * @brief @f$P(t)=e^{-r t}@f$, @f$f(t)\equiv r@f$.
+ */
+class FlatYieldCurve : public YieldCurve{
+public:
+    explicit FlatYieldCurve(double rate = 0.01) : rate_(rate) {}
+
+    double discount(double t) const override {return std::exp(-rate_ * t); };
+    double zeroRate(double t) const override {return rate_; }
+private:
+    double rate_ = 0.01;
+};
+#endif