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.hpp

@@ -0,0 +1,42 @@
+/**
+ * @file Instrument.hpp
+ * @brief Generic derivative instrument: payoff plus pricing engine.
+ */
+
+#ifndef QUANTENGINE_INSTRUMENT_HPP
+#define QUANTENGINE_INSTRUMENT_HPP
+#include "Exercise.hpp"
+#include "Payoff.hpp"
+#include "PricingEngine.hpp"
+#include <memory>
+
+class PricingEngine;
+
+/**
+ * @brief Represents a tradeable claim priced via a @ref PricingEngine.
+ */
+class Instrument {
+public:
+    Instrument() = default;
+    Instrument(double maturity, std::unique_ptr<Payoff> payoff, std::unique_ptr<PricingEngine> engine);
+    double price() const;
+
+    [[nodiscard]] double maturity() const {
+        return maturity_;
+    }
+
+    [[nodiscard]] Payoff& payoff() const {
+        return *payoff_;
+    }
+
+    /** @brief Base @ref Instrument is treated as European unless overridden by @ref Option. */
+    [[nodiscard]] virtual Exercise::Type exerciseType() const { return Exercise::Type::European; }
+
+protected:
+    double maturity_;
+    std::unique_ptr<Payoff> payoff_;
+    std::unique_ptr<PricingEngine> engine_;
+};
+
+
+#endif //QUANTENGINE_INSTRUMENT_HPP