Add numpy to build requirements for PEP 517 build isolation

The C++ extension includes pybind11/numpy.h which requires numpy headers
during compilation. Previously numpy was only listed as a runtime dependency,
causing isolated builds (pip install ., cibuildwheel) to fail with:

  fatal error: numpy/arrayobject.h: No such file or directory

This adds numpy>=1.16 to [build-system].requires to ensure it's available
in the build environment before compilation begins.

Fixes: Build failures in isolated environments
Required for: Proper PEP 517 compliance and cibuildwheel support

Author: claude

pyproject.toml

@@ -1,5 +1,5 @@
 [build-system]
-requires = ["setuptools>=61.0", "wheel", "cmake>=3.22", "pybind11>=2.9.0"]
+requires = ["setuptools>=61.0", "wheel", "cmake>=3.22", "pybind11>=2.9.0", "numpy>=1.16"]
 build-backend = "setuptools.build_meta"
 
 [project]