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/RandomGenerator.cpp

@@ -0,0 +1,19 @@
+/**
+ * @file RandomGenerator.cpp
+ * @brief @ref MersenneTwister implementation.
+ */
+
+#include "RandomGenerator.hpp"
+
+
+double MersenneTwister::nextGaussian() {
+    return distr_(generator_);
+}
+
+std::vector<double> MersenneTwister::nextGaussianVector(std::size_t n) {
+    std::vector<double> v(n);
+    for (auto& e : v) {
+        e = nextGaussian();
+    }
+    return v;
+}