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.cpp

@@ -0,0 +1,5 @@
+/**
+ * @file FlatYieldCurve.cpp
+ * @brief Ensures link visibility for @ref FlatYieldCurve (inline methods in header).
+ */
+#include "FlatYieldCurve.hpp"