nx_cgroup_v2_probe_test.nx

nx_cgroup_v2_probe_test.nx -- smoke for cgroups v2 host detection. Honest scope: this smoke MUST pass green on ANY host. On hosts without cgroups v2 (Cygwin, Windows-mounted bash, macOS, Linux with v1 only) -> available() returns 0; the smoke verifies that path. On Linux hosts with cgroups v2 -> available() returns 1; the smoke verifies the controllers file has non-empty contents. Either path is GREEN. Refusing to fail on the unavailable path is the racing-line discipline: the probe is read-only; "not available here" is a legitimate state, not an error.