Rename CPU_COUNT to NUM_CPU_THREADS so it can be overridden via env var.
Default uses _get_cpu_count() which respects cgroup limits:
- os.process_cpu_count() on Python 3.13+
- /sys/fs/cgroup/cpu.max on cgroup v2
- /sys/fs/cgroup/cpu/cpu.cfs_quota_us on cgroup v1
- os.sched_getaffinity(0) fallback
Use NUM_CPU_THREADS.value in the dataloader instead of cpu_count(),
and update export_model.py and all renderer references.
Co-authored-by: teeny-runner <runner@teeny>
