Bun: cgroup-aware AvailableParallelism / HardwareConcurrency on Linux

Summary

Routes navigator.hardwareConcurrency (and therefore os.availableParallelism()) through WTF::numberOfProcessorCores() instead of an inline sysconf/sysctl. On Linux this picks up the new sched_getaffinity + cgroup cpu.max capping in WTF, so containers with --cpus=N report N instead of the host core count.