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/Instrument.cpp

@@ -0,0 +1,18 @@
+/**
+ * @file Instrument.cpp
+ * @brief @ref Instrument implementation.
+ */
+
+#include "Instrument.hpp"
+
+Instrument::Instrument(double maturity, std::unique_ptr<Payoff> payoff,
+    std::unique_ptr<PricingEngine> engine) : maturity_(maturity), payoff_(std::move(payoff)), engine_
+(std::move(engine)){
+}
+
+
+
+double Instrument::price() const {
+    return engine_->calculate(*this);
+}
+