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/MarketData.cpp

@@ -0,0 +1,10 @@
+/**
+ * @file MarketData.cpp
+ * @brief @ref MarketData accessors.
+ */
+
+#include "MarketData.hpp"
+
+double MarketData::spot() const { return spot_; }
+const YieldCurve& MarketData::yield_curve() const { return *yield_curve_; }
+const VolatilitySurface& MarketData::volatility_surface() const { return *volatility_surface_; }