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/BlackScholesProcess.cpp

@@ -0,0 +1,24 @@
+/**
+ * @file BlackScholesProcess.cpp
+ * @brief Black–Scholes GBM drift, diffusion, and step.
+ */
+
+#include "BlackScholesProcess.hpp"
+
+double BlackScholesProcess::drift(double t, double s) {
+    double r = this->data().yield_curve().zeroRate(t);
+    return r * s;
+}
+
+double BlackScholesProcess::diffusion(double t, double s) {
+    double sigma = this->data().volatility_surface().sigma(s,t);
+    return sigma*s;
+}
+
+double BlackScholesProcess::step(double t, double s, double dt, double dW) {
+    double r = this->data().yield_curve().zeroRate(t);
+    double sigma = this->data().volatility_surface().sigma(s,t);
+    return s*exp((r-0.5*sigma*sigma)*dt + sigma*sqrt(dt)*dW);
+}
+
+