[BrendanStork]

Summary

Fixes a macOS compatibility issue in host_memory_available(), where os.sysconf("SC_AVPHYS_PAGES") raises ValueError.

The change introduces a fallback to psutil.virtual_memory().available when the system call is unsupported, and adds psutil as a runtime dependency.

The existing Linux memory-detection logic is preserved.

Testing

• python -m pytest tests/test_backend.py -v — 11 tests passed.
• CPU MPO–MPO benchmark — completed successfully on macOS.

Fixes #70

[Panadestein]

Thanks for this PR! At the moment we don't support macos for this particular out-of-core procedure. Closing for now, but we could come to it later if needed (I will leave the issue open).