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

qengine/__init__.py

@@ -0,0 +1,5 @@
+"""Qengine: quant pricing backend (native extension in qengine.qengine)."""
+
+from .qengine import bs_price
+
+__all__ = ["bs_price"]