unbounded_tail emulated Unix SIGPIPE on Windows by calling
std::process::exit(13) when a broken-pipe flush failed. Inside the
long-lived host shell that terminated the entire omp process with code
13 and no output, breaking the run contract that pi-uutils builtins
never call std::process::exit.
Flush uniformly across platforms so a broken pipe surfaces as a normal
io::Error and propagates to the caller, matching every other builtin
(e.g. uu-cat's handle_broken_pipe). Added a regression test covering the
streaming unbounded_tail path the reported repro exercises.
Fixes#5609
roboomp
d12997278d