On GitHub Actions runners, `unshare` binary exists at /usr/bin/unshare
but user namespaces (CLONE_NEWUSER) are restricted, causing `unshare
--user --map-root-user` to silently fail. This produced empty stdout
in the bash_stdout_roundtrip parity test (mock_parity_harness.rs:533).
Replace the simple `command_exists("unshare")` check with
`unshare_user_namespace_works()` that actually probes whether
`unshare --user --map-root-user true` succeeds. Result is cached
via OnceLock so the probe runs at most once per process.
Fixes: CI red on main@85c5b0e (Rust CI run 23933274144)
89104eb0a2