With fd 2 dead (a closed pane, an exited pipe reader) every write fails,
eprintln! panics on that failure, and under panic = "abort" the process
dies by SIGABRT with a crash report on the next launch instead of its
exit code. Exit reports, headless plain-format diagnostics, update-exit,
setup-result and leader-management messages now write through
best_effort_stderr::eprint_line: one attempt, failure reported, never a
panic.
Ported from upstream 1.0.19 (grok-build best_effort_stderr.rs).
Claude-Session: https://claude.ai/code/session_01RUqV8HVGXcAETyYbZ2tPQ4