Collects the core-count limits we trust and returns the smallest, never
less than one. This mirrors the (much more elaborate) min-of-signals design
of parallelly::availableCores(), but only the durable, non-platform-specific
signals: the detected core count, R CMD check's _R_CHECK_LIMIT_CORES_
(capped at two), and OMP_THREAD_LIMIT. We deliberately do not parse cgroup
quotas or HPC scheduler variables.
detect_available_cores()A positive integer scalar.