Fix PEP 517 build by adding cffi to build requirements

Add cffi>=1.17.1 to pyproject.toml build-system requirements to prevent
pip from falling back to legacy build mode. This ensures CFFI is available
in the isolated build environment that PEP 517 uses.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: electronstudio

pyproject.toml

@@ -1,3 +1,3 @@
 [build-system]
-requires = ["setuptools"]
+requires = ["setuptools", "cffi>=1.17.1"]
 build-backend = "setuptools.build_meta"