fix: respect CPU affinity (SLURM cgroups) when auto-detecting num_threads

[LeonHafner]

Summary

• Fixes #235: MetricsEvaluator crashed under SLURM (--cpus-per-task=N) when the host had more CPUs than the job allocation, because mp.cpu_count() returns host-wide CPUs and ignores cgroup/cpuset limits.
  The oversized thread count was forwarded to numba.set_num_threads() via pdex, raising ValueError: The number of threads must be between 1 and N.
• Replaces mp.cpu_count() with an affinity-aware _available_cpus() helper using os.sched_getaffinity(0), with a mp.cpu_count() fallback for macOS/Windows (those platforms don't run SLURM and have no cgroup caps in practice).
• Resolves the num_threads == -1 sentinel at the top of MetricsEvaluator.__init__ so the resolved value is used everywhere downstream rather than only at the _build_de_comparison call site.
• No new dependency required — os.sched_getaffinity is stdlib and matches the same source numba uses internally to size its threadpool.

[gemini-code-assist]

Code Review

This pull request introduces a helper function _available_cpus to accurately determine the number of available CPUs by respecting process affinity on Linux, falling back to the total CPU count on other platforms. The MetricsEvaluator class was updated to utilize this function when num_threads is set to -1. Feedback suggests enhancing the robustness of the CPU detection by handling potential OSError exceptions in restricted environments and ensuring a minimum return value of 1.

[gemini-code-assist]

It is recommended to catch OSError in addition to AttributeError. In some restricted or containerized environments, os.sched_getaffinity might be present in the os module but blocked by security policies (e.g., seccomp), which would raise an OSError. Additionally, ensuring the returned value is at least 1 is a good defensive practice for downstream tools like Numba that require a positive thread count.

Suggested change

	try:
	return len(os.sched_getaffinity(0))
	except AttributeError:
	return mp.cpu_count()
	try:
	return max(1, len(os.sched_getaffinity(0)))
	except (AttributeError, OSError):
	return max(1, mp.cpu_count())

[LeonHafner]

I think that's overengineering and doesn't really add much benefit for four new lines of code.

[LeonHafner]

Should we bump the version for this change, or just roll it in with the next version bump?

[LeonHafner]

Should we bump the version for this change, or just roll it in with the next version bump?

Since previous releases also mostly consisted of a single PR, I’ll also publish a cell-eval 0.7.2 release with this one.