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/NewtonSolver.cpp

@@ -0,0 +1,8 @@
+/**
+ * @file NewtonSolver.cpp
+ * @brief Placeholder translation unit for @ref NewtonSolver.
+ */
+
+#include "NewtonSolver.hpp"
+
+