Some Kata/microVM guest kernels (e.g. the CI runner's 6.18.x) are built
without CONFIG_PROC_CHILDREN, so /proc/<pid>/task/<tid>/children does not
exist. The Linux children() relied on it with no fallback, making
children()/live_descendants() return empty and silently turning shell
cancellation cleanup into a no-op inside such containers.
Fall back to scanning /proc and grouping by parent pid (the same primitive
the macOS path already uses) when no children file is readable; kernels
with the file keep the cheap per-task fast path. Also fixes
process::tests::descendants_includes_freshly_spawned_child under Kata CI.
can1357
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