fix: use PkgConfig::ZMQ with PUBLIC linkage and override macOS dynami…

[ww2283]

Summary

Fixes ZMQ linking issues that cause ImportError: symbol not found '_zmq_close' during editable installs.

Problem

• Editable installs failed with ImportError for ZMQ symbols
• PyPI wheel installations worked fine
• macOS Python::Module adds -undefined dynamic_lookup which defers symbol resolution
• ARM64 Mac was finding Intel Homebrew ZMQ instead of ARM64 version

Root Cause

1. Missing IMPORTED_TARGET in pkg_check_modules
2. macOS -undefined dynamic_lookup flag deferred symbol resolution
3. Incorrect architecture-specific ZMQ library detection

Solution

• Used pkg_check_modules IMPORTED_TARGET to create proper CMake targets
• Set PKG_CONFIG_PATH to prioritize ARM64 Homebrew on Apple Silicon
• Overrode macOS -undefined dynamic_lookup to force symbol resolution at build time
• Used PUBLIC linkage for ZMQ in faiss library for transitive dependencies

Testing

• ✅ Editable install now works
• ✅ Python extension imports successfully
• ✅ All ZMQ symbols resolved at build time

Related

This fix is required for LEANN PR facebookresearch#150 (metadata output feature) and PR #152 (Ollama batching optimization).

[yichuan-w]

LGTM