Respect cgroup CPU quotas for default threads

[dylanrb123]

Summary

• derive the default worker count from hardware concurrency, Linux CPU affinity, and cgroup CPU quotas
• support cgroup v1, cgroup v2, nested limits, hybrid layouts, and cgroup namespaces
• retain existing behavior on non-Linux platforms and preserve explicit thread-count overrides

Testing

• make test (11 tests passed)
• tox -e check-formatting
• clang-format dry-run
• git diff --check

Linux container validation

Compiled the CPU detector from this branch inside an Ubuntu 24.04 container and exercised it with real Docker cgroup limits. All assertions passed.

Docker limits	hardware_concurrency	Affinity CPUs	Quota CPUs	Voyager default
--cpus=0.5	18	18	1	1
--cpus=1.5	18	18	2	2
--cpus=3	18	18	3	3
--cpuset-cpus=0-1	18	2	unlimited	2
--cpuset-cpus=0-3 and --cpus=1.5	18	4	2	2

This verifies that the default uses the minimum of hardware concurrency, affinity, and the rounded-up cgroup quota instead of the host CPU count.

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