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/Payoff.cpp

@@ -0,0 +1,19 @@
+/**
+ * @file Payoff.cpp
+ * @brief Payoff function implementations.
+ */
+
+#include "Payoff.hpp"
+#include <algorithm>
+
+double CallPayoff::operator()(double S) {
+    return std::max(0., S - strike_);
+}
+
+double PutPayoff::operator()(double S) {
+    return std::max(0., strike_ - S);
+}
+
+double DigitalPayoff::operator()(double S) {
+    return S > strike_ ? 1. : 0.;
+}