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.hpp

@@ -0,0 +1,30 @@
+/**
+ * @file NewtonSolver.hpp
+ * @brief Generic Newton iteration helper (incomplete / reserved for solvers).
+ */
+
+#ifndef QUANTENGINE_GAUSSSOLVER_HPP
+#define QUANTENGINE_GAUSSSOLVER_HPP
+
+#include <functional>
+
+/**
+ * @brief Template Newton step loop with relative/absolute tolerances.
+ */
+class NewtonSolver {
+    template<typename F, typename DFinv, typename T>
+    bool solve(F&& func, DFinv&& dfinv,T x0 , double rtol, double atol) {
+        T x = x0;
+        int i = 0;
+        T increment;
+        do {
+            increment = dfinv(x) * func(x);
+            x -= increment;
+            ++i;
+        } while (i < 1000 && std::abs(increment)/ std::abs(x) > rtol && std::abs(increment) > atol);
+
+    }
+};
+
+
+#endif //QUANTENGINE_GAUSSSOLVER_HPP